Splice dumbs down GarageBand and plugs a sequencer into your web browser. It takes a Creative Commons approach to music making, so any sounds you upload are available to other users of the system, so long as you they attribute the song to you, and any songs you make can be freely mixed by others.
The site stresses a collaborative community approach, so you've got your normal social community networking hooks. It's unclear how the system deals with copyright issues - I uploaded the drums from Al Green's "So Glad You're Mine" (as last heard in Eric B. and Rakim's love song "Mahogany"). Feel free to use 'em!