Commercial music
The majority of music released under a CC license uses one of the licenses that prohbit commercial use (BY-NC, BY-NC-ND, BY-NC-SA). Below are some pointers to finding music that allows commercial use.
If your use also involves adapting the music (including syncing to video), you'll further have to exclude music under Attribution-NoDerivatives (BY-ND; which does allow commercial verbatim use), and either release your work under Attribution-ShareAlike (BY-SA) or also exclude music under that license. This leaves music under plain Attribution (BY) or in the public domain.
Jamendo
Browse Jamendo by license at http://jamendo.com/creativecommons/
One can also click "advanced search" on http://www.jamendo.com/albums to filter for albums that allow commercial use.
As of October 2010 Jamendo hosts 7414 albums under BY-SA, 1607 albums under BY, and 987 albums under BY-ND.
SoundCloud
SoundCloud's CC portal features a search that allows filtering for tracks permitting commercial use and modification. As of October 2010 the site hosted [type=&q[duration]=&q[cc_licensed]=1&q[commercial]=true&advanced=1 thousands of tracks permitting commercial use].
ccMixter
http://ccmixter.org/media/tags/attribution -- all tracks under a plain BY license. Add genre and other filters to the list by clicking '+' next to the desired filter on the right. As of October 2010, the site has nearly 5000 CC BY tracks.
http://ccmixter.org/media/playlist/browse/44 is a playlist of 100 tacks under a plain BY license chosen by the CC Creative Director in February, 20007.
Free Music Archive
FMA music search allows filtering by permission. As of October 2010 the site hosts 1627 tracks permitting commercial use.
Others
Many other sites host some music permitting commercial use, http://archive.org and http://www.dogmazic.net in particular. Please add instructions here for finding only commerce-allowed music at these sites.
Everything at http://www.tribeofnoise.com is licensed under BY-SA, which of course permits commercial use.
Everything in http://commons.wikimedia.org/wiki/Category:Sound is under free licenses that permit commercial use or in the public domain.
Libre.fm aggregates and tracks the popularity of music that allows both commercial and derivative use.
