Many sites post, or allow users to upload, music that has been licensed for re-use under Creative Commons. These sources can be helpful to find music for the background of a film or presentation, or for finding tracks to sample. Some restrictions might apply, so look at the information about individual pieces on these sites to determine what you can use and how.
"This collection hosts complete, freely downloadable/streamable, often Creative Commons-licensed catalogs of 'virtual record labels'. . . . These 'netlabels' are non-profit, community-built entities dedicated to providing high quality, non-commercial, freely distributable MP3/OGG-format music for online download in a multitude of genres."