Audiri: Find Music You Like--free!

This seems to be the newest entry in the free MP3 category (indeed, the founders were many of the former mp3.com people). I haven't had time to thoroughly try the site out, but it does have a good selection of music from a variety of genres (even comedy!), and it all seems to be in MP3 format.

They also have a good way of tracking the popularity of the bands and tracks on the site--I'll spend some time downloading the top songs to see if they're really good.