Definitely check out All Music as Tony suggested. I still tend to make up many of my own, since I'm the only one the genres have to make sense to

I use the most specific genre in the ID3 tag but my playlist has Genres and Sub-Genres (or styles, to use the All Music convention). My only genres are Metal, Rock, Pop, Electronic, International, Classical, Jazz/Blues, and Hip-Hop. I have among all these probably 25 or 30 styles (which are in the ID3 tag).

For example: I have a Metal genre, under which I have Heavy Metal, Death Metal, Grindcore, Hardcore, Alternative Metal, Thrash, and a couple of others that escape me at the moment.

That way I can listen to the entire genre or just a particular style.