What do you get when you combine your favorite anime with some hard rock / heavy metal? (No, not Hentai).
You get some damn sweet music videos! I've been watching AMV's for a long time, and they've definetly been one of my major hobbies since I logged onto the internet; at first, they weren't horribly popular, but when people saw a combination of Trigun and Disturbed, the craze skyrocketed.
Oh, and I'm not talking about that crap you can download from Kazaa (Low resolution Dragon Ball Z music videos to some Linkin Park song, sound familiar?). If you want to get a taste of the good stuff, go to some of these sites:
http://www.nhmk.com/dl.html
NHMK is some top notch stuff. I've spoken with the guy, and he puts a lot of time into his videos.
http://www.morethantoast.com/
More Than Toast productions is a group of 4 AMV making friends who usually pump out some decent vids.
http://www.kusoyaro.com/
Kusoyaro is pretty famous in the AMV field. If you like FLCL and wierd anime, he's your man.
http://www.animemusicvideos.org/home/home.php
AMV.org is a basically a site where a lot of AMV makers post their stuff, including the people I've mentioned above.