Listing of Musical Genres (UPDATED Jan 3rd, 2009)

[Saga Musix]

For electric music, Ishkur's Guide to Electronic Music is still a nice way to educate yourself. Lots of listening examples and tongue-in-cheeck comments. Hasn't been updated in over 10 years but for the classic electronic genres it works very well. Apart from that, Wikipedia often has listening examples, too.

[Mr.Eagle]

I've found this site in a German computer-tech magazine:
http://www.musicmap.info/

I think this site is entertaining and educational at the same time and there is much to discover...