We've had a couple party kills in various games.
In 2nd edition D&D, the group was wandering through a massive battlefield, trying not to get caught in the fighting (they were about level 2). The party wizard (who managed to lose an impressive 10 or so characters, all wizards, during the game) snuck off to loot some bodies and found one that wasn't dead. So, he hitches up his robes and runs through the woods, calling for help with an injured soldier chasing after him.
The party ranger sees him, aims, and rolls a 1.
I say, "make an attack roll against the wizard".
Crit.
The wizard player also inadvertantly caused the death of another character when they were fighting a black dragon. Everyone else spread out so the breath weapon couldn't hit more than one, but the wizard decides he'll be safest if he hides behind the brand-new player, who just moved to town and just finished making his first ever RPG character.
The dragon, seeing his only chance to hit more than one PC with his breath weapon proceeds to dissolve both the wizard and the new player.
The new player instantly got the nickname "Puddle," a name that stuck with him all the way through high-school, even with people who didn't roleplay - that was how he introduced himself! We never called him by his real name and I think some people in the group never even knew it, just knew him as Puddle.
In an Alternity game I ran, one of the characters was knocked unconcious by some bounty hunters and fell into a pit full of giant insect-like creatures. The characters escaped the bounty hunters and headed into the hive to find their companion.
After hours of fighting their way into its depths, they found their friend and were blasting their way out, carrying and dragging his unconcious body as they went, when the bounty hunters showed up and threw a grenade at them.
Without hesitation, one of the PCs threw their friend's unconcious body onto the grenade. I guess you might call it "involuntary dramatic heroism."
In our Dark Heresy game we played before 4e came out, we were at the tail end of a dramatic fight (the temple attack near the end of the adventure that comes with the book). My Assassin was the only one standing. The Tech-Priest had lost his leg to a cleaver and lay dead in the mud. The Skum had lost his face to one of said cleavers and lay bleeding out in a tent. The Psycher took a bullet to the chest and lay unconcious not far from where my Assassin fought three more barbarians with cleavers.
Risking cleaver attacks to get some distance, my Assassin pulls out a molotov cocktail and lobs it, missing horribly.
Rolling on the random deviation chart, the molotov cocktail lands exactly on the unconcious psyker. There was a round of high-fives after that, including from the psyker's player. (Psykers are awesome and we hate them).
The latest player kill(from my SH game) involved a PC grappled by a caged bugbear and a zealous PC archer firing into said fight. The grappled PC had about 2 hp left when the archer hit him with an arrow for something like 1d8+10 damage...
In the 4e game we're playing, we did have our rogue get possessed and drop the cleric with a sneak-attack, but he didn't kill him...