Just came across this very nicely catalogued free (creative commons) sound library website, thefreesoundproject:
The Freesound Project aims to create a huge collaborative database of audio snippets, samples, recordings, bleeps, ... released under the Creative Commons Sampling Plus License. The Freesound Project provides new and interesting ways of accessing these samples, allowing users to
* browse the sounds in new ways using keywords, a "sounds-like" type of browsing and more
* up and download sounds to and from the database, under the same creative commons license
* interact with fellow sound-artists!
We 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.