This site went around our library system a while back, and I found it again preparing for Teen Tech Camp. The site is http://soungle.com
This is what our Technical Trainer had to say about the site:
"Soungle is an easy way to search for free sound effects. Just be aware that you might want to play around with different keywords - A search for 'airplane' has only one result, whereas a search for 'plane has 10 and 'jet has another 10. All audio clips are easily downloadable."
I'm thinking of trying a few downloads, and then sharing the site with participants in Teen Tech Camp and Media Mashup.