http://www.royaltyfreemusic.com/free-music-resources.html
One subsection of the mighty Internet Archive collection, the open source audio section is primarily full of reusable Creative Commons-licensed sound files. You can also find usable audio files pocketed throughout the broader Audio collection as well.
The mission of pdsounds is to capture the acoustic phenomena in the environment of our daily lives and contribute them to the public domain for free and unfettered reuse. Search for something specific or browse the catalog of public domain sounds.
A searchable archive of field recordings from all over the world, soundtransit’s community shares recordings under the Creative Commons Attribution license.
A community of musicians and sound artists, opsound contributors upload their works under Creative Commons licenses for sharing and remixing.