Stewart,
You might look into
Freesound.org. It's a huge collection of free (natch) sound clips of all kinds of things. The clips are released under a Creative Commons license, so it's a nice resource.
Quality varies from very high-quality pro-level stuff to ghastly things that have been crudely manipulated, but you can find some gems. It's nicely tagged and indexed, so you can search based on keywords.
I've contributed a handful of things there and gotten some nice responses.
Cheers,
Stephen Hall
http://sound.facesproject.org/
