Freesound - The Freesound Project is a collaborative database of Creative Commons licensed sounds.
keezy - A colorful soundboard. Play with music.
RiffShare - Write down a musical riff, send to band members, collaborate
Figure - Propellerhead creates world-class software products and services that inspire music makers and provide the foundation for a worldwide creative musical community.
Riffr - An audio-based social network
Splice - Music creation, collaboration, and sharing made simple.