When you're trying to do a video project, such as a documentary, it's often just a labor of love. Nobody is getting paid, and there's no commercial use. However, sites like Youtube regularly scan videos for copyright violations where someone has their cat dancing to Lady Gaga, then automatically replace the existing soundtrack your choice of cheese-ball keyboard demo song. Meet your new best friend, ccmixter.org.
At CCmixter.org you can find music that's available for non-commercial use and available under a Creative Commons license. As their logo says, "download, sample, cut up, share." Some well known artists, such as Chuck D (Public Enemy), Fort Minor, the Beastie Boys, and My Morning Jacket, have contributed work that can be remixed and reused on a non-commercial basis with credit given to the original artist. Essentially, it's for guys like me who want to work on their little projects without any commercial aspirations.
The music I used for the short Geminid video came from ccmixter.org. They have a fairly simplistic search system, of which I generally use the tag cloud. Sometimes you hit the correct song, sometimes you just can't find what you want. The selection is strongly in favor of those looking for keyboard-y beeps and bops, but some acoustic music is available.
Speaking of beeps and bops...there's also mobygratis.com. Unlike CCmixter, Moby created a simple site with just four pages of song-lets that can be used free for non-commercial use or licensed for independent film. Procedes go to the human society. The music selection here is generally slow paced and electronica oriented.
I won't even get into the topic of "fair use" as a documentary film maker because I haven't found anyone who understands it. Record companies believe that there's no such thing as fair use and documentary film makers often think anything captured live is fair use. What worse is that record companies are completely blind when it comes to licensing their copyrighted songs to millions upon millions of "small" projects (wedding videos, student films, documentary shorts, podcasts, how-to's...etc). They won't even return your calls. Moby is way ahead of the curve in this respect.
So check out ccmixter and mobygratis next time you need a song for a project. Results can be quite satisfying.