OpenMusicProject is a open collaborative community of musicians. Using sofware such as Propellerhead's Reason, people can create pieces of music and upload them to this site. Other visitors can download and edit the music in any way (remixing, adding instruments, etc.), then upload the new version. Please see the FAQ for more information.

Check out some of the tracks currently being collaborated on, in MP3 format:
nuGroove, R&B Project 1, LittleGroove, lazy afternoon, roundrobin