Inspired by the ideas and ethos behind Creative Commons licensing, the tracks on the FMA are offered for whatever use you want. Use them to soundtrack your latest YouTube epic, remix them and release them, or download and share them with friends. The site also boasts a kind of quality control to the database of songs both live and recoded, selected by WFMU's audio archivists and curators. The search functionality works pretty well, and can be re-sorted by genre, album, or other criteria.
If you really dig the tunes you're finding, there are links to the artists' albums and a tip jar for each. Otherwise, stream, grab, and go at your leisure. Free to use, sign-up required for the social aspects of the site, like mix publishing and sharing.