On Angel, Lorne sacrifices his corporeal body by leaping into the Music of the Spheres, thereby restoring balance to the universe. At least he went out singing.
Spike's ultimate sacrifice in the Buffy series finale. (He gets better).
Buffy in the season 5 finale.
Ethan in Season 8. He knew he was going to die when he helped Buffy.
Giles. He got between a Twilight possessed Angel and Buffy as they fought near the Seed of Wonder, wielding the Scythe to try and destroy the Seed himself and end Twilight's invasion of Earth, or get killed by Twilight and give Buffy the motivation she needed to take the Scythe and end things herself.
In the final issues of Season 9 of Buffy, Severin sacrifices himself to help create a new Seed of Wonder and restore magic to Earth.
Merrick in both the Buffy comic and movie.
Simon Tam in Firefly was a very good one. He does not just die quickly as in most Heroic Sacrifices. He gives up his life, fortune and safety and lives the life of an outlaw just to be able to save his little sister from being tortured by the Alliance.
Topher has stepped up to join the ranks by going out with a very literal bang to save the world in Dollhouse.
Londo Mollari, one of the most complex characters in the series, saves his people from complete annihilation by allowing himself to be implanted with a mind-controlling alien symbiote.
Years later, in the future of the Time Travel sequence, Londo, under the control of the symbiote, has the heroes at his mercy. In a moment of lucidity, he lets them go, then asks G'Kar — either his sworn enemy, his best friend, or both — to kill him so the symbiote won't alert anybody to the escape. This G'Kar does, in a manner which Londo had foreseen decades earlier in a prophetic dream — which at the time he took to mean G'Kar would eventually murder him in cold blood. Londo is therefore an example of Heroic Sacrifice, Redemption Equals DeathandProphecy Twist. The symbiote then wakes up, kills G'kar, and fulfills the rest of the prophecy.
He'd also done it at the end of the Shadow War. In order to save his planet, Londo destroys the Shadow presence on Centauri Prime by blowing up their island base (including a number of Centauri who knew they were being asked to sacrifice themselves). When the Vorlon Planetkiller takes station above the planet, his sidekick Vir points out that there's still one person on the planet who's been touched by the Shadows... Londo himself. Londo orders Vir to kill him. He doesn't, but events conspire to save Centauri Prime anyway... temporarily, at least
Near the end of the fourth season, Marcus dies so that Susan can live.
Kosh deserves a mention. By getting the Vorlons to strike at the Shadows, he opened himself up to be assassinated by them.
To end the Minbari Civil War, Delenn manages to trick the leader of the Warrior Cast to join her in a ritual which will decide the outcome of the war by whoever lasts in a wheel of fire the longest. She's replaced at the very last second by Neroon who pushes her out, announces his conversion to her side, and dies, thus allowing her to reorganize the Grey Council.
John Sheridan pulls one of these when he detonates a nuclear warhead in the capitol city of the Shadows ... using his own position to guide it in.
At the beginning of "Acts of Sacrifice", an unnamed Narn cruiser does this to allow a civilian transport to escape from a war zone, interposing itself between the attacking Centauri and the jump point.
During the Battle at Coriana VI, the Shadows attempt to destroy Sheridan's White Star with two missiles. A Drazi Sun Hawk and a Minbari Sharlin interpose themselves between the White Star and the missiles, not only saving Sheridan but also showing the Shadows and the Vorlons that the younger races will no longer take their crap.
To lure the Vorlons and Shadows to Coriana VI, a shipfull of Rangers (led by Bryan Cranston!) have to die protecting a piece of misinformation, so it will be convincing enough.
Sinclair also deserves a mention, in that he travelled back in time to live out the rest of his life as Valen. Though he got to assume the role of a venerated, messianic figure among the Minbari, in doing so he left all he knew in his life as a human, with no reasonable hope of ever returning to it, leaving the familiar for an alien society in a different age. Plus, to avoid disrupting the timeline in any way, he sort of had to follow a script laid out by history. (He couldn't even use his regard as a prophet to somehow avert the Earth-Minbari War through any forewarning about the Humans.) In the final farewells before this time travel ("War Without End Part II"), clearly Sheridan and the others recognize that this is a big sacrifice on his part, some trying to talk him out of it.
Star Trek: The Original Series has a rather heart-wrenching inversion. Commodore Decker and his crew are facing an unbeatable foe, so he beams his crew down to a nearby planet and intends to go down with the ship. Except it doesn't play out like that. The enemy in question is a Planet Killer, and his ship isn't what it's after.
However, since Decker is a Four-Star Badass, he still gets his Heroic Sacrifice. But in a roundabout fashion.
Two different starships Enterprise get this in the Star Trek: The Next Generation episode "Yesterday's Enterprise". Short version: A Captain Picard from a timeline where the Federation is losing a war against the Klingons pulls a You Shall Not Pass against a squadron of Klingon battlecruisers, sacrificing the Enterprise-D in order to allow the Enterprise-C to return to her own time and get destroyed defending a Klingon outpost from a Romulan attack, cementing the Federation's status as a Fire Forged Friend to the Klingon Empire.
The episode "The Quality of Life" has one for one of the Exocomp droids (who had just been established to be self aware). Three of them enter a highly dangerous radiation field in order to allow Picard and Geordi to be rescued. However, in order to rescue the Exocomps, one chooses to stay behind to prevent the radiation from killing the other two, and it is destroyed in the process.
In the episode "Face of the Enemy", Troi is persuaded (well, forced) to act as a double agent for a member of a Romulan underground resistance movement named N'Vek, who uses surgery to disguise her as a Romulan officer of the Tal Shiar. At the climax of the episode, when Troi is about to escape back to the Enterprise after completing the mission - helping three high-level dissenting members of the Romulan government defect to the Federation - the Romulan Commander is onto her, and N'Vek threatens the Commander to buy Troi time. The angry Commander vaporizes him using a disrupter, but the stalling works, and Troi manages to escape.
In season two of The Vampire Diaries, Stefan Salvatore sacrifices himself by handing himself over to Klaus in order to get the cure for Damon when he is dying from a fatal werewolf bite. During this process, Stefan is forced by Klaus to shut off his humanity, become the blood addicted Ripper and to torture and kill innocents.
And in the third season finale, Elena insists that Stefan rescue Matt while the two of them are drowning, mirroring how Elena's father insisted Stefan save Elena before the show began.
Done rather problematically in a later episode. After having her criminally shady past revealed, the pilot Kat voluntarily exposes herself to lethal levels of radiation while guiding a passenger ship through a star cluster. The episode was well-written and actually made sense, as well as providing an unexpected degree of depth to a heretofore slightly two-dimensional character, but... given that Kat had not only served quite adequately as CAG for the last year, but was one of maybe two or three pilots who'd never actually committed mutiny during the run of the series (and that one of the others was a Cylon), would anyone other than Starbuck actually have cared that much? Might even count as Driven to Suicide. She's already taken too much radiation when she decides to fly the last mission. For her it's a choice between staying behind and probably dying anyway, or going out there, definitely dying, but making a difference, atoning not only for her past but for losing the other ship earlier in the episode.
In the BSG DVD movie Razor, Kendra, who shot some civilians under direct order of her commanding officer, Helena Cain, and was troubled with guilt over it ever since. This trope was invoked extremely obviously - someone had to stay behind and manually trigger a nuke, she forced the team at gunpoint to let her be the one to do it, and after her death, she was awarded a frakking medal. Also, the Cylon ship she died to destroy told her, specifically, that it knew what she had done and she was absolved, right before the nuke went off.
The Doctor in general tends to go out with a bang: most of his regenerations were heroic sacrifices of some kind or another. Honourable mentions go to Three (walks into a radiation-drenched cave to confront the Great One), Four (unplugs a cable with an effort that throws him off a tower and incidentally saves the Universe) and possibly Eight ( purposefully abandoning his identity to create a regeneration capable of ending the time War).
The Fifth Doctor at the end of The Caves of Androzani, which is seen as one of the best stories. Both he and Peri (a Damsel Scrappy if there ever was one) were suffering from fatal poisoning, and the Doctor gives the antidote to her. He then collapsed, and willed on by his past companions, regenerate into a new body in the best such sequence in the series.
The Ninth Doctor performed a similar feat in The Parting of the Ways, when he absorbed the energies of the spacetime vortex from his companion, Rose Tyler, so that they would not kill her. Instead, they killed him, forcing him to regenerate. (Actually, this episode has lots of Heroic Sacrifices, including the one made by the Doctor's other companion, Jack Harkness. It's kinda a cool moment as he stands there and flips off his enemy before giving a "come and get me" type gesture (he gets brought back to life, though.) Brought back to life, but also Came Back Wrong. The event made him physically incapable of deathfor a few billion years, at least. He has it tough after that.
The 10th Doctor in the second part of The End of Time. He switches places with Wilf in a box-thing which was going to fill with radiation when opened. Cue Tear Jerker as The Doctor comes out dying, and then proceeds to say goodbye to all his friends and/or companions from throughout the series. And incredibly, The Master! But then again, the viewers should have seen it coming... "Get Out of the Way!" Possibly justified, as it might means "My prey. Scram." He's too pissed off to care about anything other than making the Time Lords pay dearly for what they did to him, in the sense of The Only One Allowed to Defeat You. This is Big Damn Villains. This is the Master. And they Never Found the Body...
It has been debated whether Time Lord sacrifices count, since they can regenerate up to 12 times; at any rate, several of the Doctor's incarnations regarded it as death.
The Doctor: I don't want to go... (regenerates)
The show has developed a very specific sub-trope of its own in which an (often unwilling) agent of the Daleks betrays them and tells them off, only to get exterminated, of course.
In The Daleks' Master Plan, a desperate convict is holding Katarina hostage to hijack the ship to Kembel; he's so ignorant of Earth'shistory that he actually thinks the Daleks will help him. Rather than risk the Doctor risking all their necks and their chance to get word to Earth, Katarina blows the hatch.
In The Krotons, Serlis charges into the machine to bring the Doctor the acid he asked for. He gets it to him, but the Krotons kill him.
In Revenge of the Cybermen, one of the men carrying a boobie-trapped bomb deliberately sets it off as a weapon against the Cybermen. These frees the other victims.
Adric's death in the episode Earthshock. Ultimately, he failed in his goal (and achieving it would have been impossible without a major paradox being created), but he was trying to be heroic. Technically, he didn't fail completely — his actions prevented the entire population of the earth from being killed. Well, the human population, anyway.
In The King's Demons, Hugh insists on taking up the king's champion's gauntlet, to protect his father from having to fight the Duel to the Death. (He is quite offended when the Doctor intervenes to save his life after he fails.)
In Father's Day, after Rose destroys the timeline by saving his life, Pete Tyler allows himself to be run over by the car that was meant to kill him to restore it.
In The Family of Blood, John Smith — a fake personality created by the Doctor while hiding from some villains — sacrifices himself and dies so that the Doctor can save the day.
River does this in Forest of the Dead. Although she may be doing this to keep the timeline. If The Doctor died then he would have never met River's parents and took them on the TARDIS, thus failing in her ever being conceived.
The new series has used this trope to the point where there seems to be more episodes with it than without. Davros actually calls the Doctor out on it.
There's also Harriet Jones, Former Prime Minister, (WE KNOW WHO YOU ARE!) whose personal timeline and career are destroyed by the Doctor and still sacrifices herself to allow the Companions the time to summon the Doctor.
It's revealed that UNIT has planned this for the entire human race: the Osterhagen Keys will trigger nuclear warheads which will blow the Earth apart — to be activated if humans are reduced to such a state by some alien invasion that it's agreed they'd be better off ending it - or, as in this case, if said aliens need the Earth for some far worse purpose, and destroying it will defeat them. As shown in The Day of the Doctor, they have a similar situation set up in the event of the compromise of the Black Archive, which contains all the alien technology they've acquired. In this case, it's a nuke that would destroy London.
In the Eleventh Doctor episode Victory of the Daleks, a group of Daleks successfully enact a plan to resurrect stronger, pure Daleks. The new Daleks' feelings toward their predecessors? "You Have Outlived Your Usefulness." The old Daleks' response: "We are ready." Those Daleks took a course of action to save their race knowing full well it would end in their deaths.
Rory, who takes a shot meant for the Doctor in Cold Blood. It goes From Bad to Worse though, because his body is very near one of the Cracks in time, and it starts to erase him from existence. Even worse, Amy forgets all about him.
In Time Heist, Psi deliberately overloads his mind with the criminal records of hundreds of infamous bank robbers to lure the Teller away from Clara, figuring her ties to her loved ones make her life more valuable than his. Subverted when the "Shredder" he uses to avoid becoming lunch teleports him to safety instead of killing him.
In The Reichenbach Fall, Moriarty issues Sherlock an ultimatum: commit suicide and be remembered as a fraud, or doom his three closest friends to death by sniper. Sherlock manages to fake his death convincingly enough to both call off the snipers and walk away. However, the public disgrace remains, and he spends the next two years in exile to avoid further endangering his friends.
In His Last Vow, the only way to stop Magnusson from blackmailing Mary Watson is to kill him. Acting alone, Sherlock shoots Magnusson in front of multiple credible witnesses so that John cannot possibly be accused of the murder, knowing that he is risking arrest at best and a virtual death sentence at worst.
Eden, of Heroes, who kills herself to prevent Sylar from gaining her influencing abilities.
Then subverted. D.L. takes a bullet from Linderman to save Niki. Enter season two: he is dead, and we are lead to assume that that is how he died. But then a flashback to four months ago has him make a full recovery from the hospital, and is indeed well enough to go fight fires and stuff...only to get shot by some random crackhead with the hots for Niki.
Sons of Anarchy has Hale trying to stop a van with shooters at Half-Sack's funeral... But unfortunately, they run over his head.
In the third season finale of LOST, Charlie accepts his prophesized death and decides to go out doing something helpful. He manages to undo the jamming signal, but his real contribution is alerting Desmond to the fact that the people who have arrived at the island claiming to be rescue aren't who they seem.
In the fourth season finale, the helicopter flying from the island to a ship waiting offshore is rapidly leaking fuel. To reach the boat, it has to lose a lot of weight. Sawyer jumps out and swims back to the island.
In the fifth season finale, Juliet is pulled down a shaft, and Sawyer grabs her hand to prevent her from falling. When it becomes clear that they will both fall, Juliet professes her love for him before letting go of his hand.
In one of the last episodes of the sixth season, as most of the Losties are escaping on a submarine, it is revealed that Fake!Locke put a bomb in Jack's bag. Before it goes off, Sayid grabs it and runs as far as he can to save the others. It worked. Mostly.
The indisputably biggest Heroic Sacrifice in the series is Jack's. The Man in Black had Desmond remove a stone from the heart of the island so as to destroy the place. Jack, after being stabbed by him, still manages to get down there and reactivate the heart, thus saving the island. He then manages to get back to the same bamboo grove where he woke up in the first episode in time to die there with Vincent at his side.
In the Criminal Minds episode, "Ashes and Dust," Evan Abby lures the serial arsonist into a warehouse (which has been doused with benzene) and incinerates both himself and the arsonist. Somewhat falls under Redemption Equals Death, since Abby was the head of the company that the arsonist believed was a club for setting fires.
Vincent: "Seriously, how do you plan on getting out of here?"
On 24, George Mason (the former director of CTU) is accidentally exposed to radiation early in the second season, and slowly begins to die. He secretly stows away on a plane that contains a nuclear bomb, and is being piloted into the Mojave Desert by Jack Bauer. He ends up convincing Jack to let him fly the plane on a suicide run, letting Jack parachute out and live, as well as saving millions of innocent people.
In Season 7, a chemical plant manager called John Brunner attempts to cut off supply, when terrorists attempt to release it into the atmosphere. He instead releases a large dose into the chamber he's in, to minimize damage. Subverted, slightly, in that the attack is stopped by events elsewhere, but still, pretty damn heroic.
A moment of silence for Bill Buchanan, who set off an explosion in a gas filled room with himself still inside it to save the President's life.
And now we can add President Omar Hassan to the list, who gives himself up to the terrorists to prevent a bomb detonating in New York. They torture him and eventually slit his throat before Jack can rescue him.
A bit of a subversion in that the sacrifice, while incredibly moving and sniffle-worthy, isn't purely selfless, and Crais does take the opportunity to deliver a grand Just Between You and Me speech to Scorpius before he and Talyn self-destruct. Truly, he was a megalomaniac to the end. And we loved him for it.
When Talyn!Cricthon exposes himself to radiation to save everyone, knowing that he will die for it.
At the end of The Peacekeeper Wars, Crichton arguably knew that both the universe and he might not make it if he used wormhole weapons to stop the war. And he did it anyway. Granted, he came out fine, but he very well might not have, but he figured out that the only way to make the Scarrans and the Peacekeepers see reason—and leave him and his new wife and son alone—was to make them stare their own death in the face. And by this point, Crichton basically had nothing left to lose. He put it like this: "Either way, this war is over."
In Stargate SG-1, Dr. Jackson exposes himself to a fatal dose of radiation to deactivate a nuclear device and dies. Dying has never actually stopped anyone in the Stargate universe (and most especially not Daniel Jackson, to the extent that other characters joke about it while he's alive and wait expectantly for his return when he's not), so one year later, he was Back from the Dead.
This isn't the only time Daniel's sacrificed himself by far. He took a staff blast for Jack in the original film, pulled a You Shall Not Pass at the end of the first season (but managed to make it back), entered the virtual reality where he keeps getting "killed" to save Teal'c in Avatar, got killed by RepliCarter after stopping the Replicators long enough for the rest of the team to kill them all, prepares to send himself to the Ori galaxy with the Sangraal in order to destroy the Ori (although his friends intervene and beam him off the ship before it goes through the supergate, leaving the Sangraal onboard to activate after it gets through), and so on. This guy really likes his heroic sacrifices.
The entire team pulls this in 2010 in an alternate future. In Moebius, they all sacrifice themselves in an alternate past. Both times, they fixed it.
Samantha Carter in the alternate universe in There But for the Grace of God has a particularly awesome sacrifice, which allowed Daniel to get to the gate and back into the normal universe and stop the Goa'uld there.
Samantha: Oh, yeah. (Holds up grenade.) And I also wish to blow us all to hell.
She also pulls one in the alternate reality episode The Last Man where the 'Phoenix' is critically damaged in a battle with 3 hive ships. With most of the systems offline, she beams the remaining crew down to the nearby planet, then rams the Phoenix into the nearest of the Hive ships, destroying both vessels, and with the blast destroying the other 2 hive ships.
Dr. Janet Fraser in the series 7 episode "Heroes". She absolutely will not leave the side of a badly injured soldier on a battlefield. It saves his life, but she loses hers in the process.
In the pilot of Stargate Universethe senator seals a damaged shuttle through which they were losing oxygen, though doing so means being trapped inside and asphyxiating. His daughter got to watch. He was going to die anyway from either internal bleeding or a heart condition, so he was just choosing a third option.
In Stargate AtlantisAlternate Reality Episode "Vegas", Sheppard takes the Wraith's cash, quits his job, and leaves to live the life of a rich man (or lose it all in the nearest casino). However, on the way, he has a change of heart and manages to find the Wraith and radio his location in time for the alien to be destroyed by an airstrike before he can call in the others. Unfortunately, he himself is shot in the process and is shown bleeding severely in the middle of nowhere.
Supernatural: In the second season, John did this for his comatose son, Dean (who was surely about to die, or at least never wake up), causing the fandom to actually like him for once. Dean also did this for Sam, who died in the finale, so he wouldget better - but the fanbase are torn between this being a true Heroic Sacrifice or something to do with being Driven to Suicide. In a subversion, the show treats these as destructive, selfish acts instead of noble sacrifices.
Beyond those two big ones, Sam and Dean offer themselves up as Heroic Sacrifices on multiple other occasions. One example is Dean with the djinn in season 2. He sacrificed the ability to stay in his "dream world" - one where he doesn't have to "be a hero" and can have a normal life, including having his mother alive and his kid brother happy and not demon tainted - after realizing that in that world, all the people who the Winchesters have saved over the years are instead dead. The means for "returning" to the real world? He had to die. (Which could very easily tie this one back into Driven to Suicide, but this is * Dean* we're talking about; the two often aren't exactly far from each other when it comes to his motivations!)
Sam's season 4 Heroic Sacrifice which is averted when his own attempt to sacrifice his soul to save Dean is rejected. His resulting decision to damn himself slowly by using his powersseems like a noble act - until you get to the end and realize that Sam's efforts have been fueled by demon blood that intensifies pride in his own abilities and a sense of superiority to the point where he believes he is The Only One who can stop the Apocalypse. While Sam is genuine in his desire to help his brother and save the world (see When The Levee Breaks for his huge inner conflict), he goes about it in averydestructive(especiallyself-destructive)way. He can't know that killing Lilith starts the Apocalypse; the angels hid that so he would kickstart the Apocalypse by doing what he thought would prevent it. The goal is noble, even with the horrible outcome. It's part of the show's deconstruction of this trope that everything turns out so badly.
Gabriel. He decided that he was on humanity's side (eventually); went up against Lucifer and tried to kill him (even though he still loved all his brothers and left because he hated watching them fight); gave them a DVD which told them how to put Lucifer back in the cage and was killed by his brother. Particularly heroic as he told them before that he'd 'skipped ahead' and knew how it would all end, so it's very possible that he knew that he would be killed and still did it because his death would be the only way that Sam and Dean could save everyone.
In Prison Break, Brad Bellick, who spent the first two seasons as a main antagonist and the third as a pain in the arse before joining the heroes in season 4, sacrificed himself to ensure the grand plan would be completed. They make a point to show the character's dead body by the time the episode is out, just to be clear.
The series finale has Michael Scofield sacrificing himself so the final prison break can succeed and his wife and unborn child can be free. However, the nosebleed implies that his condition was not cured and he was going to die anyway.
JAG: Gunnery Sergeant Granger dies during the street ambush in "War Cries".
In Power Rangers in Space, Zordon, who, after being held hostage and drained of energy for a year, begged the man that had finally arrived to rescue him to stab his can, which was done after some convincing. The resulting magical explosion destroyed the enemy army throughout the universe, turned three villains human, and brought the rescuer's recently deceased sister back to life.
The viewers' response to this was so overwhelming positive, that the writers felt they were safe using such a plot element again; in Power Rangers Lost Galaxy, Kendrix sacrificed herself to save Terra Venture from Psycho Pink. (In this case, it was done because Valerie Vernon had to leave the show after being diagnosed with leukemia. Kendrix did get better in the finale of the series.
Kamen Rider Raia, being a fortune teller, predicts the death of Shinji Kido/Kamen Rider Ryuki. But he sacrifices himself to avert his prophecy, because he knew Ryuki would a better warrior than himself.
Kamen Rider Tiger, after realizing how his Chronic Backstabbing Disorder made a mess of his life, jumps in front of a truck to safe a father and a little kid.
Ironically, the main hero does this too. Shinji dies protecting a little girl from a monster.
Way before them, Riderman in Kamen Rider V3 saves Japan from a nuke, apparently being taken with it, leading to V3 to declare, officially, that he was the fourth Kamen Rider. He shows up in Kamen Rider X no worse for the wear, though, and Kamen Rider Spirits reveal that he did survive, but suffered amnesia for awhile.
Subverted in a comical manner in Kamen Rider Gaim. Kamen Rider Bravo throws himself in the line of fire to save Kamen Rider Zangetsu-Shin from an attack by the powerful Over Lord Inves Deemushu, causing him to revert into his human form. It may seem dramatic with Bravo lying weakly in Zangetsu-Shin's arms, as if Bravo is going to succumb to his injuries in the arms of his lovely warrior in white. Then it turns comical as Zangetsu-Shin nonchalantly throws Bravo out of his arms and out of harm's way. He got better, though.
In Flash Forward, FBI agent Al Gough learns from his flash forward that his actions at some point result in the accidental death of Celia, a woman he has never even met. He also knows that fellow agent Demetri Noh had no flash forward and has been told he will be murdered. He chooses to sacrifice himself by jumping from the roof of the L.A. FBI building to prevent Celia's death and to show Demetri and others that the flash forwards can be changed.
Sadly this proves to be pointless, she ends up dying anyway.
In The Outer Limits episode "A Feasibility Study", aliens who have been immobilized by disease abduct a whole neighborhood of humans. The people are informed that if they can survive the harsh environment of the aliens' home world, then the rest of the human race will be abducted and turned into slaves. The humans decide to deliberately infect themselves with the alien disease to save Earth, causing the aliens to cancel their plans for mass abduction.
In the episode "Better Luck Next Time", the two protagonists defeat two evil Body Surfing aliens by killing the host bodies and then themselves. Too far away from any other people, the aliens die.
"Summit" has an almost literal example when the sole survivors of a peace summit offer to kill themselves to prove their sincerity and ensure that the peace treaty they negotiated before terrorists murdered the other representative party was accepted.
"The Voyage Home" has the protagonist blow up his spaceship to prevent an evil alien from reaching Earth.
In Fracture, Chloe uses her healing powers to bring Lex, who is pretty obviously a villain at that point and has done her great personal harm previously, back from the brink of death at the cost of her own life. Luckily, she has Resurrective Immortality, although she remained dead for 18 hours. Clark is worried the next time she won't be coming back.
Deconstructed in the episode "Lazarus". Clark Kent finds himself in the afterlife after dying in the previous episode to stop Zod. Jor-El calls him an idiot for not realizing that even though he did save the day, he's now left Earth vulnerable to future threats. Fortunately, Lois manages to revive him.
In the first episode "The Duel", Midshipman Clayton takes Horatio's place in a duel and as one episode guide described it, "becomes the first in a long line of people to cheerfully die for Horatio".
In the episode "Retribution", Archie Kennedy takes sole blame for a mutiny in order to save Horatio from being hanged. It wasn't his life he was sacrificing — he had been wounded in a battle shortly before and was dying anyway — but his good name and the pension for his surviving family members, suggesting how loyal he is to Horatio.
Writers must love writing this trope for Chronically Killed ActorJamie Bamber. Aside from Archie Kennedy, Tony Dewhurst (the very first of his characters to be killed off) was shot and killed trying to protect The Scarlet Pimpernel from being gunned down (adding insult to injury is that Dewhurst lived throughout the series of books the miniseries is based on), and the most recent one, DS Matt Devlin of Law & Order: UK was shot protecting his friend Alesha Philips (along with the young witness in their case) from a hail of gunfire.
In the Sliders episode "The Exodus", Professor Arturo steps in front of Quinn Mallory and takes a bullet meant for him. At this point Arturo is already severely brain-damaged thanks to Colonel Rickman, but he still sacrifices himself so that Quinn can live to get the others home.
In Community episode "Epidemiology" Abed allows himself to become infected in order to allow Troy time to escape.
On Burn Notice, Ian sacrifices himself to stop a rogue diplomat.
A few episodes later, Fiona turns herself in to make Anson's leverage over Michael worthless.
In the Grand Finale, Madeline Westen goes out with a bang in order to protect her grandson and give Michael a little more time to stop James Kendrick. "This one's for my boys..."
Season 2: Stephen locks himself in with Helen's captured creatures to trap them, in among the most dramatic and heartwrenching scenes in the entire series. Cutter offers himself up for one multiple times throughout the episode, but is constantly stopped at the last minute by other characters. This episode is the one most often regarded as the best by fans and critics.
Series 5 Episode 6, a.k.a. the finale: Phillip Burton realises that his anomaly chanelling power station is out of control. Matt is with him in the control centre when Phillip tells him to leave, and that he will shut down the machinery holding the artificial anomaly open. He says, "Hey, I'm the bad guy." But it isn't enough. He shuts down the machine, but the artificial anomaly no longer needs it, and it is large enough that it destroys the building. Phillip is killed as the building collapses. This is distinct from a Redemption Equals Death because Phillip neither intended the destruction, nor did he die following rather than during it.
Matt Anderson does one in the climax of the episode, but... he lives.
Septa Mordane stands in front of Lannister soldiers to prevent them from taking Sansa. It costs the woman her head.
Choudenshi Bioman: After being hit with a ray that was already starting to kill her Mika/Yellow Four 1 took the rest of the shots saving the other four team members as well as rendering the weapon useless.
Last Resort: Captain Marcus Chaplin makes one of these to make sure the Colorado and its nuclear weapons do not fall into Chinese hands and explicitly forbids his second-in-command from joining him.
The Walking Dead: In season 3 episode "Killer Within" there are two: first, T-Dog (who was already bitten and was as good as dead anyway) threw himself at two zombies so Carol could escape (which she did), and then Lori forced Maggie to give her a C-section, even though Maggie had no experience with the procedure, to save her baby that couldn't be born naturally. After a tearful goodbye with her son Carl, Maggie cut her open and saved the baby.
House of Anubis: In the finale of the second season, Nina is willing to send herself into the afterlife with Senkhara, in order to release the curse off of her friends and family. She doesn't end up leaving, though, as Senkhara was taken away by Eddie.
In the same scene, Joy has one. Nina (who was possessed by Senkhara at the time) threw a lightning bolt at Fabian- and she jumped in his way to save him. She didn't die, though, as Victor sacrified his last tear of gold to save her life.