In Victory In The Pacific, this is the usual result of sending a single ship up against a large enemy force to take out the lone enemy patrolling vessel, or to destroy the lone enemy amphibious force.
During the Clan Invasion in BattleTech, Tyra Miraborg rammed her crippled aerospace fighter into the Clans' flagship, the Dire Wolf, striking the ship's bridge and killing ilKhan Leo Showers. This act forced the Clans to delay the invasion for a year while they elected a new ilKhan and bought the Inner Sphere much needed time to regroup and rearm before the offensive resumed. Tyra actually had no clue that she was attacking such a vital target, she just knew that her fighter was too badly damaged to survive and got extremely lucky with her Taking You with Me.
In Dungeons & Dragons Oriental Adventures, Honor Points can be awarded to a character posthumously (which benefits future characters created by the player) and dying heroically in this manner gains such characters a lot of them.
In Shadowrun just after his inaugural speech President Dunkelzahn rips out his own heart in an effort to stop the effects of Blood Magic and is assumed assassinated.
The Dungeons & Dragons supplement Book of Exalted Deeds advises DMs to go easy on resurrection penalties for good characters who go out on one of these; there's even the Risen Martyr prestige class.
The Imperial Guard, period. Their entire existence is a heroic sacrifice. Against a galaxy crawling with monstrous Tyrannid bugs that outnumber them by an exponentially large ratio, the Lovecraftian horrors of Chaos, the Orkish hordes, life-sucking skeleton-men, mutants, psychics, barbarians and technologically advanced aliens... they're basically conscripts with a ridiculously obsolete lasgun in their hands and some flak armor. They're cannon fodder for the Space Marines who can't be everywhere at once. Unlike the Space Marines, they're not elite super-soldiers with demigod-like training and equipment, just regular people living in a dangerous and uncaring universe full of superpowered evil. And, despite this, they Hold the Line.
The Apocalypse Reload book gave them a new strategic asset: Fire On My Coordinates. Choose a soldier in your army with a communications backpack and have a cruiser in orbit fire a torpedo/plasma blast right on top of them (though they have to pass a morale test to do it). You can just see the squad crouching in the trenches, most of them desperately holding the enemy off while the main army retreats, the communications trooper shouting "fire on my coordinates!" into the microphone, blasting the advancing Ork/Tyranid/Chaos/etc. horde and the squad into very small pieces.
Also important to mention is the Imperial Guard Saint Ollanius Pius, the guardsman (or Terminator Captain, or Custodes, Depending on the Writer) who threw himself between the Emperor and Horus, even though he just saw Horus tear apart Sanguinius, arguably the greatest of all the Primarchs. Ollanius was rended from existence by Horus' sheer power, but it was this final act that finally convinced the Emperor that Horus was beyond saving.
Speaking of, Sanguinius. Wounded, exhausted and having seen his own death in the future (yes, Sanguinius was able to divine the future to a certain degree), resisted Horus' promises of power, wealth, fame and (most importantly) life in favour of certain death to inflict a chink in Horus' armour which the Emperor would later exploit to bring him down.
The final stand of the Astral Knights chapter. To wit, a Necron World Engine (think the Death Star piloted by Omnicidal Maniac killbots) was flying straight towards Terra, and the Imperial Navy along with some Space Marine ships were dispatched to destroy it. Every attempt to destroy it failed due to powerful shielding, and the Imperial Navy took heavy losses. So the Astral Knights fly their battlebarge Tempestusstraight into it at full speed, punching through it's shields and smashing into the surface. Seven hundred and seventy two Space Marines poured out of the wreckage with the intention of destroying every cannon, power node and command module they came across, beset on all sides by tens of thousands of Necrons. The battle lasted for a hundred hours, and ended only when the Chapter Master himself and five other warriors penetrated their way deep into the World Engine and destroyed the main power nodes with melta-bombs, bringing down the shields and allowing the Navy to blast the World Engine to kingdom come with huge volleys of cyclonic torpedoes. After the battle a monument was erected on the planet Safehold, constructed of the wreckage of the Tempestus and guarded by volunteers from other chapters who were present at the battle, wwith 772 statues of Astartes each representing the Astral Knights marines who gave their lives to protect the Imperium. You can cry now.
Older fluff has one story of an Eldar force who covered the escape of an Imperial contingent, including civilians, from a massive horde of Tyranids. The Imperials managed to escape successfully, but the Eldar were wiped out.
After the defection of Primarch Magnus the Red to Chaos, Malcador the Sigillite was the only man left in the Imperium of Man who had the power to temporarily replace the God-Emperor on the Golden Throne, to allow the Emperor to battle Horus. Being a mere mortal, Malcador was fully aware that it was a task that would end his life, but it was necessary to protect the human built sections of the Webway from a massive daemonic invasion. When the mortally wounded Emperor was brought back to the Golden Throne, Malcador the Sigillite mustered the last of his power to temporarily revive the Emperor, allowing him to dictate to Rogal Dorn plans to turn the Golden Throne into an arcane life support system. This allowed the Emperor to keep himself in a state of near death for over 10,000 years to maintain the Astronomicon and protect humanity from the Warp. Malcador's body turned into dust after this last act.
Most Space Marines have badass or threatening Battle Cries. Not the Lamenters chapter. For them, it's "For those we cherish, we die in glory", and they mean it. When the Lamenters launch an assault on an Ork stronghold world, they won't nuke the planet and call it a day; they will save all three million slaves on the planet first, or die to the man trying.
In TORG, one of the subplot cards the players can use is Martyr which, once set up, will allow the player to automatically succeed at something by heroically sacrificing their own life.
Averted actually with Abel in Anima: Beyond Fantasy. While his death, crucified by Solomón, after thirteen hours of agony and lanced with what is an Expy of the Soul Edge seems to Expy Jesus Christ's death, except the part of being killed with a weapon designed to kill gods, everything was actually planned by Imperium to have him dying as a martyr, so people would follow his teachings as they wanted.
Traveller, Double Adventure 5 The Chamax Plague/Horde. In Horde, the planet Raschev is suffering an Alien Invasion by voracious and quick-breeding creatures using Generation Ships. When a pilot sees an alien ship landing, he flies his plane into it, damaging it and causing it to crash. The ship is damaged enough so that almost all of the aliens aboard are killed and the survivors can be easily disposed of by the PCs. His heroic action prevents the invaders from gaining a foothold that would have ended in the deaths of the entire human population of the planet.