In El Goonish Shive, Nanase experiences the pain of death repeatedly (through the repeated destruction of her fairy-doll avatar) during her fight with Abraham for Ellen's life. She has to be hospitalized afterward due to the extreme expenditure of magic and loses her magic completely for over 8 months.
Erfworld: Bogroll taking down Lord Ansom himself. There's no possible greater glory for the lovable oaf.
Fleep. After discovering a very nasty truth about his past actions, Jimmy gives up his chance of being rescued in order to assure that another civilian would survive. Jimmy's own internal monologue downplays the heroism of the sacrifice—he just wants to see his wife, and now that she's dead, redemption is the only shot he has at ever seeing her again.
Magick Chicks: Faith has put herself on the line for her fellow students on two occasions:
General Tagon (Captain Tagon's father) comes at this from a strategic angle. If you're going to sacrifice someone, it should be someone you care for—if it's someone you hate, it's not a sacrifice, it's murder. This ends about as you'd expect.
Captain Kevyn: You have many friends, general? General Tagon: Alive? No, not really.
Maxim 20: If you're not willing to shell your own position, you're not willing to win.
Maxim 70: Failure is not an option. It is mandatory. The option is whether or not to let failure be the last thing you do.
Sidekicks: In the season 1 finale, Metheos recovers during his fight with Darkslug and fires a Beam Burster at Dream Girl. Darkslug puts his body in the way of the attack (mirroring an event that occurred in Dream Girl's youth) and takes grievous damage as a result. But Metheos charges up another full body Beam Burster to finally destroy Darkslug for good. Before he can, Darkslug tells Dream Girl why he picked her as his sidekick, transfers all his powers and the First Prana into her and then shields her from Metheos' explosive blast with his hero cape.
Homestuck: There are a number of these throughout the comic, somewhat mitigated by the fact that every player starts out alternating between two bodies so that they can be resurrected into the other if one dies, all players go to an afterlife from which they cans till interact with living people and each other, and there are a number of ways to cheat death beyond that. In that regard, these sacrifices are more similar to sacrificing an extra life or body in a video game than actually sacrificing one's life in real life or other works. This is not to say that characters actually know that this is the case when they sacrifice themselves, mind.
At the end of Act 4, Jade sacrifices her Dream Self to push John out of the way of an oncoming meteor.
In Act 5, "Hivebent", Sollux withstands the Mind Rape of the cry of Eldritch Abomination that was killing of every troll on the planet just long enough to get Feferi into the Medium.
Sollux has another go at it when he overexerts his powers to propel the asteroid to the green sun before their window of opportunity passes.
This is one of the only two ways that a player who has ascended to the god tiers can die a permanent death, the other being a Karmic Death.
During the Escape from Brassmoon story arc of Goblins, a group of monsters is holding off guards by the gate while waiting for it to open. It finally does, to their relief and joy, only for a group of guards to charge towards them. The ogre in the group pushes the others towards the exit, grabs a spear and goes to meet the guards. Two pages later, you see two of the guards stabbing him repeatedly.
Subverted in the same arc a few pages later. Complains tells Big Ears that he's badly injured and can't move, so Big Ears will have to—and Big Ears cuts him off, furiously telling him he won't leave him behind. Complains stares at him for a moment, then:
Are you high?! I was going to say you should find a board or something to put me on, and you could maybe drag me out of here! Damn right you're not leaving me behind!
Played with in the case of Chief, who stayed behind to fight Kore to allow the rest of his group a chance to escape and hide in a dungeon crawl. The other goblins decide to go back to save him after they hear his screams of pain, but they utterly fail to do so.
In Powerpuff Girls Doujinshi, Atomic Betty was about to get crushed by a giant robot claw (they never showed the robot), then Sparky runs and pushes her out of the way so he could get crushed to death instead. Leaving Betty with a big Say My Name.
Tales Of Gnosis College: Moira makes one in hopes of preserving friendly relations between humanity and Howard the Tentacle Beast's species. Nanetta makes one for Moira at the same time.
The Adventures Of Dr Mcninja. In the "Futures Trading" story arc, Chuck Goodrich was protected from far by the NASAGhast, which killed any of the dinosaurs that tried to harm him. But the battle was being lost due to the dinosaurs' ultimate weapon, the Horrorsaurus. In a moment of honor and courage, Chuck provoked the Horrorsaurus into attacking him so the NASAGhast would kill it, but the spirit didn't act quickly enough to prevent his gruesome death.
Later he does it again in Such Stuff... to save Sarah from Jareth and Jareth from himself and this time he actually dies. But this one was All Just a Dream combined with Your Soul Is Mine by Jareth's father. So he got better.
Dragon Ball Multiverse: According to the South Kaioshin, the Grand Kaioshin did this in most universes to tame Majin Buu.
Guilded Age: A badly injured Rachel pushes Frigg out of the path of the reality-destroying monster the cultists summoned rather than trying to escape.
At the end of Nan Quest, a Sadistic Choice is revealed: either Pablo or Santiago must die. Pablo sacrifices themselves to spare Nan from having to choose.
Xenol in Second Empire develops an extremely ingenious plan to cripple the pursuing enemy ship following their disastrous escape from Skaro. However, he needs explosives, and the closest thing he has is the Self-Destruct Mechanism in his warriors' travel machines. Fully knowing just how important Xenol will be to the Daleks, all present warriors immediately volunteer. Xenol proceeds to show his awesomeness by ensuring the two chosen ones survive the Self-Destructive Charge.
In chapter 11 of Unsounded, Elan throws himself in front of the enemy forces to ensure Elka makes it to safety; he doesn't last long before the enemy wrights crush him between pillars of earth. The villain scores some Irony points by calling him a coward as he does so. It's implied that he did it because he was going to die soon anyway, as he had a heart condition and had gone several days without his treatment.
Angel Moxie: Multiple times. Grant does this early on to defeat Shugari's demon minion. Later, at the end of the story, Alex does this to defeat Yzin, and Miya does this to save Alex.