(on weeaboo sites and chatrooms) to lol. The "w" comes from "warau" (笑う), Japanese verb for "laugh". The actual number of w's used is immaterial but 5 is considered to be a common minimum.
Haruhi: Previews are worthless, listen to my song!
Kyon: BOMBERRRRR!!!
Random Weeaboo: wwwwwwwwwww