What was Leeroy Jenkins? I will tell you.
In the fall of 2005, when World of Warcraft was aswingin' a guild decided to do some quest runs. When they came upon a cave, filled with enemies they could fight, but would probably lose against, they decided to fight. The leader of the group, came up with a good plan of attack, that if executed probably would've done a fairly good job. But, that's not who we're talking about.
The man behind the name Leeroy Jenkins was doing something during the planning session. Accounts vary from listening intently to the plan, taking a piss, taking a dump, taking a smoke, or microwaving chicken (mmmm..rubber), however whatever he was doing, when he hit the ctrl key on his keyboard (which, for those of you not attuned to the way of Teamspeak, activates the mic), he would vault himself into immortality.
The Leader of the group asked, "Abdul, crunch the numbers for me quick."
Abdul: "Sure, give me a sec... Thirty-two point three three, repeating of course, chance of survival."
And then the man hit the button. It's been debated by the people who heard it what was more epic. The:
"Alright chums, let's do this. LEEEEEEEERRRRROOOYYYYYY JEEEEEENNNNKKKKIIINNNNSSSSS!"
or the following:
"Oh my god, he just ran in."
After the man ran into the cave, the team followed after him, and were promptly slaughtered.
That's nice, sure, but most often forget that the Leeroy Jenkins piece was actually a Machinima video on the Laughing Skulls PVP server done by Ben Schultz. Blizzard Entertainment was able to market merchandise of it, but it wasn't very successful.