Heroic Sacrifices are a staple of Superhero books (since, it's what makes them Superheroes and not just Supers), and a main cause of Comic Book Death.
Kingdom Come: After spending most of his teenage and adult life under Lex Luthor's mind control, Captain Marvel dies by taking Superman's place and making a nuke detonate in mid-air, saving countless other superheroes. His death sends Superman on a Roaring Rampage of Revenge.
In a particularly convoluted example, Lightning Lad sacrifices himself in battle with Zaryan the Conqueror, and every member of the pre-Crisis Legion participates in what amounts to a lottery to determine who will sacrifice their own life to bring him back. Saturn Girl, his Love Interest, cheats to make sure hers is the life sacrificed - but she is in turn replaced by Chameleon Boy's shapeshifting pet Proty, which sacrifices itself in her place.
Ferro Lad sacrifices himself to destroy the Sun-Eater.
Chemical King dies preventing the start of World War VII.
Karate Kid sacrifices himself to save his wife's home planet.
Non-lethal version: Matter-Eater Lad saving the universe by eating the otherwise indestructable Miracle Machine, but driven insane in the process. (The writers were really looking for an excuse to write him out of the series; his powers were only useful at close-range, which was a handicap on a team where everyone could fly. Letting him go out as a hero made it more dignified.)
Magnetic Kid dies to unlock Sorcerer's World during the "Magic Wars" storyline, in an effort to prove himself to his older brother Cosmic Boy.
Leviathan, thanks to a Literal Genie granting his "heart's desire" to die a hero, sacrifices himself to stop Dr. Regulus in the post-Zero Hourreboot.
Live Wire (the post-Zero Hour version of Lightning Lad) resigns from the Legion and sacrifices himself to save his friends and Love Interest from former-teammate-turned-Omnicidal Maniac Element Lad in the limited series Legion Lost.
Heroic sacrifices seem to be a sort of pattern for Lightning Lad; don't be surprised if it happens again in the Threeboot.
Similarly to Supergirl, Superman continues fighting Doomsday to the death to protect the people of Metropolis, even though it meant sustaining mortal wounds in the process. He was Only Mostly Dead.
In the first issue of the current series of Justice Society of America, Mister America is introduced, a patriotically-themed super-detective... who has no problem beating up suspects. His family is killed by a villain to destroy his legacy. He shows up to beat the tar out of the villain... and then he gets mortally wounded. His response is to run from the Boston dockyards to New York's Battery Park (using Le Parkour), jump through the Justice Society's skylight, and hit the table in the main meeting hall, dying on impact. In response, the Justice Society tracks down his family's killer.
According to writer Tad Williams, if Aquaman Sword of Atlantis had not been canceled, the second Aquaman, Arthur Joseph, would have sacrificed his life to revive the original Aquaman by giving up the piece of Aquaman's soul that had revived Arthur Joseph. This was so that the original Aquaman could fight an evil entity only he was capable of defeating. Then, perhaps poor Arty Joe would have gained a little more sympathy from fans.
Astonishing X-Men: Colossus infected himself with the Legacy Virus a few years earlier, thereby releasing the cure into the air deleting Legacy from existence. But it's ok, he got brought back to life in Astonishing.
In Astro City, the Confessor sacrificed his "life" and reputation to stop an alien invasion — the reputation because the sacrifice revealed that he was a vampire, and made him appear to be a serial killer.
Captain America's death was like this. Wearing power dampening handcuffs, Cap notices an infrared sniper pointer on one of his captors. Being the selfless man he is he throws himself in the line of fire and promptly gets shot. This was the villain's evil plan to begin with, so here's one for the bad guys.
Mr. Immortal from the Great Lakes Avengers did a Heroic Sacrifice in the end of Issue #4 by committing suicide. Since his power is to return from the dead, that wasn't that heroic, or much of a sacrifice to begin with. Doorman on the other hand let himself die by getting Mr. Immortal to that very place, but he returned to life as some sort of angel of death.
X-Man ended with Nate Grey destroying his body, becoming Pure Energy, and merging with every organism on the planet in order to prevent a Planet Eater alien from harvesting all mitochondria on Earth (From the alien's perspective, Nate's energy contaminated the "crop").
In the X-Wing Series comics, it's revealed that Wedge's parents sacrificed themselves when a pirate set their fueling station on fire by going in with extinguishers to slow down the fire before it could get to the tanks, allowing enough time for everyone else to get away. Wedge is horribly, calmlyfurious at the pirate in question.
While fighting the reborn Emperor and his forces, the Skywalkers came across a cyborg Jedi Knight named Empatojayos Brand who had survived the Purge (barely). He followed them and helped defeat Palpatine, but, as the Sith Lord had transferred his soul from cloned body to cloned body to escape death, he launched his spirit at the infant Anakin Solo. Brand interposed his damaged body in between Palaptine and Anakin and used the Force to bind Palpatine to him as he died, thus ridding the galaxy of the Emperor forever.
In the Gargoyles comic series by SLG, a female gargoyle dies after shielding her mate from a rain of arrows. Fittingly, she is known as "Sacrifice" in the script.
In the last comic of Earth X, the Celestials had landed on Earth and is getting ready to destroy the planet. Tony Stark knew that he had to buy time for Galactus to arrive to fight. So, he went up in a giant Iron Man suit and fought off a bunch of Gods by himself.
And prior to that, while the Celestials were still in orbit, Black Bolt attacked them, using his powers at maximum to call Galactus, which shatters his body.
In Empowered, when the Space Station is falling from orbit, Mindf*** sacrifices herself so Emp can go through the portal instead. (Emp tried to sacrifice herself so that Mindf*** could go through instead, but Mindf*** used her psychic powers to make Emp go instead. Sistah Spooky tries to save Mindf*** using her magic, but fails.)
In Teen Titans #74, Eddie Bloomberg, formerly known as Kid Devil (before he was depowered), is given his heroic send off. While the other Titans are fighting the Fearsome Five, Eddie discovers a dying metahuman with nuclear powers about to have a meltdown which would destroy all of San Francisco. Without any hesitation, Eddie takes the living nuke to the Titans jet, being horribly burned and subjected to lethal amounts of radiation in the process. Then he takes the jet into low Earth orbit, refusing to eject just to make sure the jet doesn't change course and go back down to Earth. Eddie vanishes in a nuclear explosion, with the Titans' battlecry "Titans Together" as his last words.
In Fantastic Four #587, Johnny Storm sacrifices his life to make sure an army of Annihlus-like creatures never escaped the Negative Zone.
A recent issue of Amazing Spider-Man has J. Jonah Jameson's own wife, Marla, sacrifice her life to save her husband. The action and resulting death is so powerful that, for probably the first time in his life J.J. can't bring himself to blame Spider-Man for something that was his own fault!
In Justice Society of America, three generations of Hourmen try to do this. In the original timeline, the first Hourman Rex Tyler sacrificed himself to stop Extant from destroying the universe. Time shenanigans by the third Hourman (an android from the future) allow the first one to spend time with his son Rick the second Hourman, knowing that he will eventually have to go back to the point of his death and sacrifice himself. Rick refuses to let his father die since he wants his mother to have the same chance to reconnect with Rex that he had, and tries to take his father's place. In the end, the android Hourman makes the sacrifice.
In V for Vendetta, it's Rosemary Almond, a mere housewife whose whole life has been destroyed by Norsefire who decides to sacrifice herself (and avenge herself), by killing Commander Adam Susan, the state's dictator, knowing full well what would happen to her if she did so. She is then mobbed by a horde of Fingermen. Her final fate is unclear, but it's probable that she doesn't survive.
Tommy Turtle, having been body jacked by A.D.A.M., allows himself to be destroyed by the Egg Fleet.
Sir Connery sacrificed his life force to destroy the corrupted Crown and Sword of Acorns.
Knuckles' father Locke killed himself in order to break the spell that had brainwashed Knuckles into becoming Enerjak.
Sally Acorn caused herself to be roboticized to prevent Eggman from doing it to the whole planet.
Antoine D'Coolette grappled with a Metal Sonic and pulled it away from the transport holding Elias and his family, letting Dr. Eggman detonate it and kill him instead (he survived, though he is currently comatose).
Ultimate Spider-Man himself does this with style, taking a bullet to the stomach meant for Ultimate Captain America, then racing back to rescue his friends and family from the escaped Ultimate Sinister Six before he finally succumbs to his wounds.
The UK Marvel Transformers comics featured one of these mixed with Crowning Moment of Awesome. Inferno is injured in his ship above a huge battlefield. As he lays dying he remarks "Always wanted to go out in a blaze of glory" and then promptly crashes his ship into the heart of the Decepticon forces, wiping out a majority of them.
This is almost a tradition for Optimus Primes in the various continuities. In at least two instances, once each during the original Marvel run of comics and once during the Dork Age release in the 90's, Optimus plunges himself and the Autobot Matrix into two different types of Planet EaterOmnicidal Maniac. It turns out that Evil Cannot Comprehend Good and the result is volatile enough to bring salvation in the form of massive explosions. Optimus Prime being Optimus Prime, his death rarely 'keeps' for long, though.
In Transformers: Last Stand of the Wreckers, Pyro has actually been diagnosed with a condition known as primus apotheosis, which makes the sufferer idolize Optimus Prime, and try to make themselves as similar to him as possible, including his penchant for heroic, noble sacrifices. Subverted with Pyro's being torn to pieces by a horde of Decepticons as a distraction, and averted with the entire story, which is about "Good people dying in stupid, pointless ways".
In Transformers: More than Meets the Eye, after Overlord is set lose on the Autobot Ship The Lost Light, Rewind sacrifices himself by crawling into the slow room where Overlord is and unjamming the door, leaving Rewind at the mercy of Overlord. Lampshaded by Rodimus, who believes it is a cheap way to make amends for wrongdoings, considering he was the one who brought Overlord onto their ship in the first place. He also risks his own life to stop the signal from killing those who were constructed cold.
Jericho Drumm, aka Brother Voodoo, sacrifices himself as his final act as Sorcerer Supreme, rebelling an invasion by a major entity calling itself Agamotto.
In the "Day of Vengeance" leadup to Infinite Crisis, the last Lord of Order (The Spectre already killed the others along with the Lords of Chaos) Nabu engages The Spectre in a battle that he knows he can't win. The Spectre murdering the last Lord of Order convinces the Presence (which is effectively God in the mainstream DCU) that he's gone out of control, and The Spectre is promptly sealed into another mortal host.
In Secret Six, Tarantula is killed after telling Junior she has the Get Out Of Hell Free card. Scandal has it.
During the Spider-Man storyline Spider-Island, Mr. Fantastic confronts Eddie Brock, at this point as Anti-Venom, and tells him his symbiote is the cure to the virus that's giving people Spidey's powers and turns them into the Spider Queen's army of humongous spiders, but if he does so with so many people the Anti-Venom symbiote would die. Eddie's response? he goes to a church and gathers everyone infected and cures them. It counts as a Heartwarming Moment because, after the symbiote dies, Eddie realizes that, for the first time since his hell began, he finally became the hero he saw himself to be. He was the hero of Spider-Island.
In W.I.T.C.H. Luba, who had been sceptical of the Guardians and tried various times to get them replaced, saves them and an almost dead Caleb from Nerissa, who is now practically invincible and helped by four creatures that can actually give the Guardians a run for their money, by attacking her and her minions, knowing well that she had no chance to survive. She somehow kept all five of them at bay long enough for the Guardians to escape and bring Caleb to safety.
One could probably say that the event that supposedly turned Jean Grey into the Dark Phoenix was this. After all, she never expected to survive the radiation, and did it simply to save the rest of the team. In the original storyline, she did survive, but was turned into the Dark Phoenix which led to the whole Dark Phoenix Saga. After the whole thing was retconned and the Dark Phoenix was revealed to be a separate entity posing as Jean, her original action seemed more fitting of this Trope after all. (She was Only Mostly Dead due to the intervention of the Phoenix Force.)
In My Little Pony Micro Series Issue #2 Rainbow does a Sonic Double Rainboom to help inspire everypony in Equestria and dispel the gremlins' cloud. However, doing so leaves her unable to fly for two months.
X-23 does this. A lot. In Target: X she's prepared to willingly return to the Facility with Kimura in order to spare her cousin and aunt (Kimura decides to torture them to death anyway to punish Laura for escaping, forcing Laura to fight back). She attacks Nimrod head-on to draw its attention away from the other kids and takes a direct blast of its weapon, which overloads her Healing Factor and she only survives because of Hellion's intervention. In X-Force she takes the Legacy Virus into herself and is about to throw herself off a building to destroy it, and only survives because of Elixir's intervention. In Avengers Arena she makes a frontal attack against Apex (who is now controlling a Sentinel) to try protecting the other kids, and only survives when Apex grabs the Idiot Ball. Significantly, Laura is borderline suicidally depressed and has a low or non-existent sense of self-worth due to her abusive upbringing, which drives her willingness to sacrifice herself for others.