I get amazed by how many genres of music are out there now. The electronic music world is a great example: House, Deep House, Trance, Psy-Trance, Techno, Drums & Bass, etc. I'm glad there in not a test on these...

There is a company in Denver called Beatport  that has done an excellent job compiling a huge selection of music for DJs, clubs and anyone interested in electronic music.

You can kill a few hours just sampling the tracks and there is a good chance you end up filling your crate before leaving their site.