Spider-Man: The climax of The Clone Saga saw a badly beaten Ben Reilly jump in front of the Goblin Glider when the Green Goblin tries the same trick how ramming the Goblin Goblin into Peter that he tried in The Night Gwen Stacy Died, with Ben dying as a result. The 2009-2010 Alternate Continuity miniseries saw a Morally Superior Copy of Norman does this instead when Harry tries the same thing.
Superman has done this a few times and is glad to do so for the innocents he can shield with his own body, but being Immune to Bullets, it's not quite as dramatic.
The Death of Superman: In The Return of Superman, the Eradicator does this for Superman, and as the bullet in question is a ridiculously huge dose of green kryptonite, it's thoroughly dramatic.
At the end of the 1941 "Scientists of Sudden Death" arc in the newspaper comics, Ralph Roland attempts to shoot Superman. He fails because Lil Danvers jumps in the way and dies. This sacrifice lacks justification, as Lil had, on prior occasions, seen Superman get dumped in a vat of chemicals, stabbed, and repeatedly get caught in bridge-destroying explosions to little apparent effect.
Kryptonite Nevermore: Superman dives in front of a torpedo heading towards a freighter. Since he has lost most of his power it's thoroughly dramatic. The explosion doesn't kill him but it leaves him dazed.
Supergirl has also done this fairly often. In Supergirl (2005) storyline Girl Power, Kara does this for Batgirl when Poison Ivy mind-controls several cops into shooting her friend.
Subverted in Supergirl: Cosmic Adventures in the 8th Grade when Superior Girl fires her Eye Beams at Supergirl, and Streaky leaps into the path of the blast. However, Belinda's Superior Vision mutates Supergirl's pet, turning Streaky the Supercat into Streaky the Super-Saber-toothed tiger.
In The Hunt for Reactron, Flamebird throws herself between Supergirl and Reactron's Kryptonite blasts.
In one Superman's Pal, Jimmy Olsen, Jimmy volunteers for an experiment which sends him back in time to experience the lives of men who match his genetic structure (effectively reincarnation). He discovers to his horror that one of his previous incarnations ran away in battle, dooming Richard the Lionheart. However, Superman, who used his own time-traveling abilities to check the incident, tells Jimmy that "Sir Oliver James" actually saw a traitor aiming a fireball at Richard. He died taking the blow for the king.
"The Super-Steed of Steel": During an alien invasion, Comet blocks a Kryptonite beam aimed at Supergirl. Justified because, unlike her, Comet is not vulnerable to Kryptonite.
In "The Unknown Legionnaire", villain Norm Eldor attempts to gun four Legionnaires down, but Unknown Boy flies into the way and shields the group of his gunshots.
The Avengers: Princess Ravonna does this for Kang, after deciding she really did love him (despite the whole "conquered her entire kingdom" thing). At least some versions of Ravonna died because of that, some didn't. Fast forward a few decades later, and another Kang (maybe) steps in front of an oncoming Mjolnir for Ravonna. Later still, another Kang found himself wondering what the hell that was all about, and figures he'd gotten suicidally bored.
Booster Gold does this for Blue Beetle. The bullet passes though his goggles harmlessly but they let you think that he's dead for a second.
In Batgirl (2009), Cassandra Cain takes several rounds while reaching a gun-wielding thug rather than dodge and endanger... another of the gang she and Batman are facing. Thus taking Thou Shalt Not Kill to a new level.
Captain America: Cap himself does this in "The Death of Captain America". Being marched to a courthouse after Civil War, he sees a sniper drawing on the officer in front of him. Being Captain America, he leaps in front of the shot... which was the Red Skull's plan all along.
Sabretooth does this for Monet in Uncanny X-Men (2016). He & Monet are on a mission to find & protect Elixir. Sabes, sensing their attackers, tackles Monet & Elixir to the ground. Elixir starts to get up, but Creed continues holding on to Monet, shouting in pain while he's shot in the back.
In the ElfQuest series Kings of the Broken Wheel, Cutter takes two arrows meant for the High One Timmain. He survives, but not without difficulty.
The first Crimson Dynamo, Anton Vanko, does an inverted version of this for Tony Stark after defecting from Russia and working for/becoming friends with him. Vanko deliberately fired an unstable, experimental laser pistol at the second (and evil) Crimson Dynamo in order to kill him before Dynamo could kill Iron Man, even knowing that doing so would probably mean his own death as well. It does.
Has happened at least twice in the comic book version of W.I.T.C.H.. During the fifth saga, Cedric gets in the way of a beam which would've killed Orube, and in the seventh saga Liam is killed by a sword that the White Queen threw at Mariko.
In the Chick Tract "Murph," Murph's partner, Officer Donovan does this for him, since he believes he, being an "unsaved" Catholic, isn't ready to die. Interestingly enough, Donovan gets off with flesh wounds while Murph is mortally wounded.
In Birds of Prey a young teleporter calling herself Batgirl (complete with homemade Batgirl outfit) teleported herself in front of a bullet meant for Huntress. Fortunately, her teleportation came with a Healing Factor and she teleported herself to Barbara in time to save herself.
Iron Man: Tony Stark does this far too often for good sense during battles, especially for Captain America. Granted, his protective armor does give him some logical basis for this, but he still frequently winds up badly injured.
In Ultimate Spider-Man (2000), we have the eponymous Spidey doing this to save Captain America from a sniper's bullet. He dies, yes, but not before going out in a blaze of glory by fending off the Sinister Six. After taking a bullet for Cap and being left to be found by paramedics, Spidey starts making his way to a hospital. Unfortunately, he realizes the Sinister Six are going after his family and friends, so he makes a detour to save them. After repeated hope spots and barely managing to fend off and defeat the Sinister Six while bleeding out, he is actually rescued by Aunt May and Gwen Stacy. But, after seeing Norman Osborn get back up, Peter uses the last ounce of his strength to stop Osborn and save his family and friends before bleeding out on the front lawn of his own house with all of Queens watching. So one physical bullet and dozens of metaphorical ones.
In Sonic the Hedgehog (Archie Comics), a heart-broken Mina Mongoose throws herself between Princess Sally and Sonic (who had just reconciled and admitted their feelings) and a bullet from Nack the Weasel. The comic makes it seems she died, but then she's seen in a hospital room recovering.
In Uncanny X-Men, Rogue took a sustained laser blast meant for Wolverine and his bride Mariko (this after Wolverine had said to her face that he would have liked nothing better than to cut out her heart for what she had done previously to Carol Danvers). The blast was enough to nearly kill even her; since she won his respect by this sacrifice, Wolverine risked his own life (he had been poisoned and badly wounded earlier on in the issue) to let her absorb his Healing Factor.
Towards the end of both continuities of Life With Archie: The Married Life, Archie saves Kevin Keller's life from assassination by getting in the line of fire and charging at would-be assassin and busboy Wendell... and ends up taking the bullet for Kevin before dying.
Civil War (2006): Taskmaster, a member of the Thunderbolts, is about to shoot the Invisible Woman while her back turned so she wouldn't know to block it with her force field powers. He at the time estranged husband Mister Fantastic jumps in the path of the bullet so fast that he can't use his own stretching powers that would keep the bullet from hurting him and he falls to the ground in pain. Sue responds by flattening Taskmaster into the ground in one move as revenge.
Wonder Woman (1942): Antiope redeems herself for betraying Hippolyta by jumping between her former queen and a blow that would have killed her in the final battle.
Wonder Woman (1987): When a mobster she's set to testify against sends a mook to kill Officer Isabelle Modini the junkie that put her in the hospital in the first place is in her room trying to apologize to her while she belittles him, and he jumps in between her and her attacker, saving her life but dying at the foot of her bed.
Young Avengers: Patriot takes a shot from a Kree blaster meant for Captain America much to Cap's horror. Steve is all set to donate his blood to save Eli, but Eli's grandfather Isaiah beat him to it.
In Superman/Shazam!: First Thunder, there is a supernatural variant when Superman and Captain Marvel meet for the first time when they are fighting monsters who have a magical attack, which Superman has no special resistance to, and Cap helps Superman freeing him when he is hit. When the second monster tries the same attack, Captain Marvel stands his ground in front of Superman and easily wards off the attack since his powers are based on magic and the gods, which impresses Superman.
In the comic book, Megatron takes the shot that was intended for Cliffjumper. Before that, Cliffjumper was not really sure he can trust the Decepticons of Mirror Universe, but Megatron's act of heroism was what finally convinced him to fight on their side and help them defeat the evil Autobots.
Played for Drama in the prose story "Transhuman". When Seaspray fired a missile at Sephie Beller, one of the Decepticons' human allies, Shockwave jumps in to protect her with his own body, but nearly dies from the missile. This results in Sephie developing Survivor Guilt, which made her undergo a process that would turn her into a Cyborg so that no one would ever take the shot for her ever again.
The original Black Canary's husband Larry Lance died protecting his wife from a laser.
The Legend of Zelda: A Link to the Past (1992): Inside Ganon's Castle, the party is swarmed by ghosts of Link's past foes, with a specter of Agahnim leering dangerously at Zelda. Roam quickly switches to eagle mode and charges at the illusion, electrocuting himself.
The Transformers (Marvel) has Impactor, leader of the Wreckers, taking a cannon shot for Emirate Xaaron and dying in his arms... though not before naming Springer as his successor and calling Xaaron a "wily old buzzard."
. Even more awesome than other examples because she has been warned those bullets can hurt her