Ishkur's Guide to Electronic Music is an online Flash-driven guide to electronic music created by Kenneth John Taylor of British Columbia. Current version (2.5) was released in January 2005 and lists 180 genres. Of course, there may be some problems with accuracy (especially mp3 audio demos) but still it's probably the biggest guide available. Version 3.0 is in progress.
More info: Ishkur's Guide to Electronic Music