The Primer compilations are an introduction to the latest Open releases. Each batch of new music will include a preview download PLUS some bonus tracks from other friends of Open. They have very good selection of releases, you must go check some stuff !
Launched in September 2008, Open is an open source music label run by musicians:

« We operate from Brighton, UK. Open is run by Piratio but is greatly helped by John Cohen & Leo Piggot. We don’t believe in the conventional record label. OpenMusic’s objective is to release great music and sound projects from around the world, in a freel, share-friendly and re-distributable manner. We encourage you to redistribute individual tracks or whole releases via sharing links, Peer-to-Peer networks, MP3 Blogs and any other form you see fit, as long as you respect the rights of the respective Artists and follow the rules of the Creative Commons Sharing License. »