This is a search engine built around sound files found on the web.  It's pretty cool.  Of course the content may be copyrighted, so what you do with those sounds, you do at your own risk. 

You can specify various properties of the sound such as file type, sample rate, size, etc.  They have a piece of software to augment the task as well.

If only they would let you search by copyright status too. 

Go find sounds at: http://www.findsounds.com/