SoundBible

A huge catalog of sound effects. Very clear attribution for all the sounds (which are mostly CC BY and public domain).

http://soundbible.com/

“SoundBible.com offers free sound clips for download in either wav or mp3 format. We offer free and royalty free sound effects and clips for video editors, movie scores, game designers, and weekend sound warriors. Bookmark our page now by hitting Ctrl+D. We update weekly so be sure to return for one of the best copyright free sound resources on the net. Downloads are totally free and upfront with large download buttons to prevent confusion.”