Freesound is a huge collaborative database of sounds
- not songs or compositions - but sounds: audio snippets, samples, recordings, bleeps.
All sounds uploaded to the site must be licensed under the Creative Commons Sampling Plus license 
The Music Technology Group 
and the Universitat Pompeu Fabra 
, have created The Freesound Project 
which provides new and interesting ways of accessing these samples...
* Browse the sounds in new ways using keywords, a "sounds-like" type of browsing and more
* Up/Download sounds to and from the database, under the same creative commons license
* Interact with fellow sound-artists!
They also aim to create an open database of sounds that can also be used for scientific research. Many audio research institutions have trouble finding correctly licensed audio to test their algorithms. Many have voiced this problem, but so far there hasn't been a solution.