If you need background music for a video or slideshow, you could try the Free Music Archive.
The music is arranged by genre – Blues, Classical, Country, Electronic, etc. – and then sub-genre, such as Rock > Loud-Rock > Noise-Rock > Sludge. (More suitable, perhaps, as background music are genres such as Instrumental > Ambient.)
Tracks can be downloaded as mp3 files and then added to your project. Most of them have Creative Commons licences such as Attribution-NonCommercial-ShareAlike.