Star Trek: The Next Generation: In "Yesterday's Enterprise", the Enterprise is being attacked by three Klingon warbirds; Riker states that they can outrun them, but Picard orders the ship to stand fast and fight the Klingons so as to allow the past Enterprise to escape. Prompts the famous line, "Let us make sure history never forgets. The name. ENTERPRISE!"
Kor was a legendary but disgraced Klingon, managing to survive so long as a warrior he was going senile. In "Once More Unto The Breach", he took command of a damaged vessel, and lead a skeleton crew of volunteers on a suicide mission to delaying a Jem'Hadar squadron from overtaking the Klingon task force they were chasing. This was deemed so impressive by the Klingons on the ship he'd been serving on (even Martok, who hated the man) that they all broke out in song, including Martok . Truly a warrior's death.
In "Blaze of Glory", Eddington single-handedly masterminded a scheme to save his people AFTER his whole Face–Heel Turn—not to mention being captured and imprisoned by Starfleet—and then went down fighting to ensure that everyone escaped.
Variation of this occurs too many times to list in Star Trek: Voyager. Janeway attempted to sacrifice Voyager on a number of occasions, and at least one version of Janeway succeeds. Dubbed a "variation" because she's also willing to claim "You Shall Not Pass" on behalf of the crew and ship, as well as herself.
The Battlestar Galactica is being pummelled to oblivion by Cylon Basestars....when suddenly the far more badass Battlestar Pegasus shows up and starts stomping on the Cylons. The ship is destroyed taking out the last Cylon ships so that Galactica can escape. Possibly the most Awesome Moment of Awesome on that show.
"Tomb of the Cybermen". The Doctor and Jamie attempt to close the electrified doors to the tombs, sealing the Cyber Controller inside. When they realize that doing so would kill them as well, even if they could overcome the Controller's attempts to force its way out, the enormous Toberman pushes them aside, declares, "You shall not pass Toberman: The door is closed," and forces the doors shut, killing himself and (apparently) the Cyber Controller.
In the new series episode "Tooth and Claw", a character who had previously betrayed the Queen repents, and so of course has to die, so he volunteers to sacrifice himself to give the main characters more time to escape the werewolf, despite the fact that normal humans with guns had previously been shown not to slow it down at all. His death is not shown directly, so it is not known if or how he actually slowed it down.
Done by the Captain of the Queen's guard earlier on in the same episode. Despite being told by the doctor that bullets have no effect on the wolf he calmly reloads his revolver and says "You do your duty [find a way to kill the wolf] and I'll do mine" then steps around the corner to face it.
In the new series episode "Doomsday", a Cyberman, formerly the director of the Torchwood Institute, turns on its cybernetic fellows at the last possible moment before they break in on the Doctor & Rose and stop them saving the day; this rebellious Cyberman even says "You will not pass" twice, and then mows down the other Cybers with an electric rifle. It's actually a very touching moment. Then comes the single black tear; borders on Narm but we'll let it pass.
In the new series episode "Dalek", one of the soldiers tries to hold off a fully-charged Dalek with a pistol. We then see a flash of light, the sound of a Dalek shot and a scream. Then we see the Dalek approaching.
Part 3 of "The War Machines" back in the William Hartnell era. In the last scene of the episode, a rampant robot is bearing down on the (characteristically) useless army and random civilian director-type-people. They all turn and run for cover, but the Doctor stands his ground in what could be more of a CMoA too.
In the episode "The Stolen Earth" in the recent series, the American UNIT general and random Red Shirt single-handedly stand against an oncoming Dalek force to give Martha Jones time to teleport away. ... and of course, get lasered rather quickly. I mean, come on, these are Daleks.
In "The Wedding of River Song", we get Rory versus The Silence. The device that lets him remember them is booby trapped and causing him agonizing pain, but he keeps it on so he can remember them and cover the escape of Amy, the Doctor, and River Song once the Silence break in. Unfortunately, it gets too bad for even him once the juice gets cranked up. The Silence still don't get to pass, because Amy, who was told to run, comes back with a machine gun.
Marcus Cole does this to Neroon in the episode "Grey 17 is Missing". Clips over part of the episode show Marcus getting his ass kicked. While he is prepared to meet his death to make this delay as long as possible, Neroon spares his life. Neroon then immediately concedes that Delenn is the proper leader of the Rangers by saying "They would die for you; I don't think I could make them die for me."
The exact lines from Marcus: "I am a Ranger! We walk in the dark places no others will enter! We stand on the bridge, and no one may pass! We live for The One! We die for The One!"
In another episode, in a vision of a possible future, Garibaldi stays behind to make sure Sinclair can escape, even going so far as to declare 'This is the moment I was born for!'
Subverted with the Battle of the Line. The Minbari rolled into the Sol system, effortlessly trampling the human fleets and bases arrayed against them, and on their way to Earth with the intent to Kill All Humans. The President of Earth made a passionate speech urging any volunteers to fight to the bitter end and buy time for others to flee Earth to neutral territory. While the Minbari were generally impressed by humanity's general Determinator spirit throughout the one-sided war, they only called the attack off and made peace with humans when on examining one of the fighters, a man named Sinclair, the Minbari discovered, to their horror, that some humans had Minbari souls, completely blowing their Ape Shall Not Kill Ape thinking out of the water.
Subverted in the first season two episode of Stargate SG-1 in which Daniel is badly wounded. Thinking that this will likely kill him anyway, he tells Jack to take the others and go on without him while he plans to stay behind and cover them. However he ends up meeting no resistance and instead hauls himself to a Goa'uld sarcophagus to get healed.
Played straight (or perhaps homaged) in the episode "The Enemy Within" where Teal'c beats a Goa'uld infested Major Kowalski to the gate room before he escapes, utters "You cannot pass" and kills him on the ramp.
Made ever more awesome because of the circumstances. "The Enemy Within" is the second episode of Stargate, and at this point Teal'c's fate is yet to be decided (One option was to remove the infant Goa'uld within him for study, which would kill Teal'c). The Goa'uld inside Kowalski takes him over, opens the Gate to Chulak, and enters the gate room to find Teal'c blocking his way. Keep in mind that the Goa'uld are Gods to Jaffa. The Goa'uld orders Teal'c to stand aside. Teal'c: "You cannot pass."
"Heroes": Government agents are attempting to capture Micah, because he's been helping other heroes to safety. Finding themselves in a parking garage, Micah and Tracy, who recently learned that she's his aunt, seem to be cornered. However, Tracy tells Micah to run, and also to activate the sprinkler system (Micah can "talk" to machines). He does so, and we get a Crowning Moment of Awesome for Tracy, as she activates the full power of her freezing ability once Micah's out of range, freezing half of the garage, killing the agents, and sacrificing herself in the process.
Of course, we later find out she is still alive, after pulling a T-1000 melt-and-reform maneuver.
Supernatural: Castiel does this in the Season 4 finale to hold off the prophet Chuck's archangel so that Dean can reach Sam.
In the Season 5 episode "Abandon All Hope," Jo decides to do this by building a bomb to hold off a group of hellhounds because she's mortally wounded. Right as the plan is about to come to fruition her mother, Ellen, decides to join her, reiterating the show's theme of the importance of family.
In the Season 8 episode "Taxi Driver", Friendly Neighborhood Vampire Benny, who had befriended Dean in Purgatory and hitched a ride back to Earth with him earlier in the season, allowed himself to be killed by Dean in order to rescue Sam from Purgatory, promising to come back with him the same way he did with Dean. However after they find the back door out of Purgatory, they are attacked, and Benny opts to stay behind and hold them off. However, it's made pretty obvious Benny would have stayed behind anyway, since Warm Bloodbags Are Everywhere on Earth, and he didn't want to hurt anyone.
An episode of Gomer Pyle, U.S.M.C. entitled, They Shall Not Pass has Gomer paired with Sergeant Carter. When Carter is injured, only Gomer is left to protect a bridge, which he does successfully.
24 Season 8, Episode 14: A Secret Service Red Shirt stays behind to fight off the attacking military squad while Jack & Co. attempt to evacuate President Hassan.
As in its source material, the show features Syrio Forel holding off a Kingsguard knight and several Lannister mooks with a wooden sword while his student Arya escapes. However, we never directly see a killing stroke inflicted on him, and his head is not amongst the ones adorning spikes on the wall (like Ned's or Septa Mordane's), so whether he's dead or not is left ambiguous.
And later Yoren tries to do the same for Arya and Gendry. It doesn't work.
In another off-screen example of this, Grenn, along with five other Watchmen, hold the Wall's gate against a freaking giant! Sadly, none of them make it.
Bran, worged into Hodor, holds a door closed to prevent a horde of undead from breaking through. It caused Hodor's seizure in the past, rendering him unable to say anything but Hodor, shortened from "hold the door". Hodor's last scene shows the wights breaking through and tearing at him.
Jorah Mormont does it twice, first against Viserys when he tries to steal dragon eggs and later against a battle-hardened Dothraki fighter. Fittingly, his family motto is "Here We Stand."
Summer dies holding the line in his attempt to prevent a horde of White Walkers from getting to Bran.
Primeval. Captain Becker tends to do this. Quite a lot, actually. He survives each time, and each time he does so with a Big Damn Heroes moment. In case you haven't heard, Captain Becker's kinda awesome.
Green Arrow attempts this in a "bad alternate future" in Smallville, grimly notching a Kryptonite arrow and facing down an incoming platoon of superpowered soldiers. The outcome isn't shown, but it's a fairly safe bet it was a Dying Moment of Awesome.
Subverted twice in Person of Interest. In "Firewall". Reese stays behind to battle a hit team while the POI escapes. Of course that's exactly what she wants, leaving her free to kidnap Finch (who's waiting with the escape car) without his interference. Turns out she's the sociopathic hacker Root. In "Shadow Box" Reese gets arrested by the FBI due to the exact same circumstances of staying behind to fight off the opposition.