artistspaid:

From rifflet:

A rifflet is a piece of a song—a bass line, a guitar riff, a drum beat or something else entirely. Share your rifflets and combine them with others, or even upload a finished song.
Here’s the catch: every rifflet must be less than 60 seconds. The idea is to only upload a part of a song, or an idea for a song, or just a couple of guitar chords you think are cool. Then, you can download parts of other people songs (like a drumbeat) that you can combine with your ideas.
If you’re a dj, you can use all these great pieces of proto-songs for remixes without worrying about royalties or copyright restrictions, because these sounds are all on this site for the purpose of being remixed.

 There is another site (currently a private beta) called MixMatchMusic that is offering a similar service, but they will be tracking each sample, and promise to pay royalties for contributions to songs featuring your samples.