Www, not to be confused with the one for the World Wide Web, likely originates with the Kanji character for "laugh," 笑, which is pronounced as "warai" in Japanese. "Warai," in message boards and chat rooms, quickly became shortened to "w" as an indication of laughter. And then, much the same way "ha" begat "haha" begat "hahaha," the sentiment became extended -- to "ww" and then "www" (and also, if you're so inclined, to "wwwwwww").