This word originates from a mangling of the interjective Woo (which itself came from Whew). Its popularity began to rise with the prominence of online multiplayer games--in particular, online role-playing games such as EverQuest?. This is probably as a result of the phenomenon of online communication morphing words to create strong statements that are easy to type. See also -- grok, blorf?, editors, feel free to add some other new-age jargon that have interesting roots
W00t is the lamer? version of woot. In the language of the lamers, certain letters are replaced with numerals or mathematical symbols to create an elitist alphabet. W00t is one variation of woot, with woo+ being yet another variation.