I joined HitRecord.org 2 years ago and I've found it an interesting place to collaborate with other people. They do a film festival each year with collaborative shorts (some are currently under production). The platform was founded by Gordon Joseph Levitt and if your work ends up being used in a final product you'll receive money for your work. You can participate by uploading your work, or by participating in open challenges or by starting your own project. All this with the understanding that someone will be able to remix your work if they feel like it (also, you'll be able to remix anything in the platform).

They have released books, tv shows, commercials, short films and music. Let me know if you join.