Freesound is a project underways to create a database of audio samples released under a Creative Commons Sampling Plus license. In other words, they want you to be able to download and use these samples for almost any purpose you see fit, as long as you include information about the original author.

Sounds are organized by the now ever-so-popular tags system, and can also be rated by users. And since one of the most obvious uses of a sample is to make a remix, there is also a remix tree that allows you to view the different generations of remixed samples. There is even an API called Mootcher, available in C++ and Pure Data flavors.

Freesound is an initiative from the Music Technology Group at Universitat Pompeu Fabra, Barcelona. Support them by giving them your samples.