Ace of Spades said:
My favorite is "yourself" in CoD4 multiplayer. "You killed yourself".
Also works well in left 4 dead and perfect dark, especially in perfect dark; kill message for a player killing you would be "You killed yourself" and if you kill them the get the message "Killed by yourself".
A few I've come across and used myself:
crunchy-pubes
MyLilPwny
Mathemagicks
PsychoTheRapist
edit: almost forgot, saw a guy by the name of G.I. Jew: The All-American Hebrew on left 4 dead
another guy by the name of Saddaminator who would message you saying "you were Saddamized!" after he killed/attacked you.