rocbattle probably gets more exposure then any of them, but don't expect to get neptunes money off of there. MySpace is ok for that as long as you have enough friends to bulletin
http://www.mkufubeats.com is a brand new site with less competition where you can upload your beats for free and start selling them for free , no commission or any monthly fees
There are a lot of game designers looking for music other than the typical royalty-free stuff. You may want to try FGL.
How about using soundcloud? many production or producers know about that place.? I know a software that well help you produce beats like hiphop, trance, dub etc etc...