Wikipedia says:

Leeroy Jenkins, sometimes mis-spelled Leroy Jenkins and often elongated with numerous additional letters, is an Internet phenomenon named for a character created by World of Warcraft player Ben Schulz in Blizzard Entertainment's popular MMORPG. The character has become popular thanks to a video of the game that circulated around the Internet. The phenomenon has since spread well beyond the boundaries of the WoW? community into other online games and media.

This Internet phenomenon started with the release of a video clip online to the World of Warcraft forums. The video was released by the World of Warcraft Alliance player guild PALS FOR LIFE on the Laughing Skull PvP? realm. The video was intended as a hoax, and in some ways blatantly makes fun of "nerd-guilds" which plan battles extensively. (WikiPedia:Leeroy_Jenkins)

See also: http://www.youtube.com/watch?v=LkCNJRfSZBU&mode=related&search=

citation, quotation --jmax, Wed, 18 Apr 2007 19:00:18 -0700 reply

I edited this slightly to make the attribution clearer. We'll get you guys citing and quoting like scholars yet, darnit.