Extensive site dedicated to the bully on "The Simpsons" has pictures, sounds, a biography and a Nelson Muntz quiz.
Browse an episode guide to the bully's appearances, see screen grabs of him, and listen to his characteristic "ha-ha" jeer.
Info on Nelson has a listing of episodes in which he stars, as well as all the times he says "Ha ha!" Also lists times he has punched Bart, Milhouse, Martin, and others.