I discovered Opsound when it was mentioned over at Creative Commons. There was nothing more than an invitation to participate at Opsound for a long time; but the site is now filling up with free and legal MP3s. I have not listened to any of it yet though.

Opsound is a record label and sound pool using an open source, copyleft model, an experiment in practical gift economics, a laboratory for new ways of releasing music. Visit the Opsound Open Pool for free copyleft music downloads. Musicians and sound artists are invited to add their work to the pool.

… you are free (and encouraged) to download works, make copies, share them, include them in other works, remix and rearrange them, and even sell them.