Free CC-licensed Sounds

Soundsnap is a "free online sound library and community for sound designers, musicians, producers, filmmakers, web/game designers and anyone else who needs sound effects and music loops," according to Tasos Frantzolas, who runs the site. "The sounds are a mixture of professional content and user-uploaded sounds."

The site currently hosts 40,787 samples in categories including Exteriors, Nature, Transport, and of course Music Loops (the most popular), all available under a Creative Commons license that lets anyone use and transform the samples for commercial and non-commercial uses (only selling them on a collection of samples is forbidden).