Here are some of my favourites.
1) This one's great: The Pink Side of WOW (it starts after about 30 seconds)
http://www.youtube.com/watch?v=j3H2_MoUBA8
2) Here's my favourite Ranma AMV: Gimme Dope Johanna (a parody of "Gimme hope Joanna by the excellent German group JBO). Knowledge of German is required(if you need a translation of the lyrics just ask me)
http://www.clipfish.de/player.php?videoid=MjA1NzY0fDEy
3) Here's a great classic everyone should have heard about: All Your Base (I love the song)
http://www.youtube.com/watch?v=qItugh-fFgg
4) The gay referee
http://www.youtube.com/watch?v=9WUTM_rA5NA