soundcloud.com Lots of free and low-cost music on this site from independent musicians and sound artists

freemusicarchive.org Lots of free music by Indie artists in many categories

Wikimedia Commons This is a large repository of media of all types that is offered for royalty-free public use.

Vimeo has many videos tagged as “creative commons” which are royalty-free for use.

freesound.org This site has tons of free sound effects and ambient textures.

The YouTube audio library (which you can find from your account within YouTube) has royalty-free music for mixing into your project.

Smartsound.com also has royalty-free (mostly very commercial-sounding) music for sale.

Audiosocket.com and Soundtaxi.com have royalty-free music tracks, and both companies can track down licensing for other music as well.