exclamation of joy, origins much-contested, but definitely pre-dating online games such as Counterstrike and Everquest- oldest usenet post containing such usage is January 08 1996. Said to originate from 133tspeak (which would pre-date that post by several years), an exclamation of 'root!'- meaning someone had achieved root-level access in a Xnix-based system, and thus, had full access.
"I'm required to enter an example! w00t!"