For some games I’ve been using background music in games. I generally don’t want to fiddle with computer while gaming, so optimally I construct a playlist that the computer can play in the background as we happily roleplay. The problem, of course, is finding or building suitable playlists. Luckily, one of my friend is pretty good at finding suitable background music, even if he prefers fairly epic music.
Mood and so on are the normal cited benefits of background music. I also use it as an improvisational tool: A significant change in the tone of background music is a good cue for making similar change in gameplay. Works as long as the music doesn’t change dramatically too often.
Arcana and Dargaard are great for dark and fairly ambient background music. Darkwave is the genre, I think, but I’m not expert on those. There is chanting, melodic music and rarely singing.
Ambient in general is fairly usable; for example, check Dreamstate.
Of course, sometimes someone else has done all the work and you can simply play it. Last.fm is free for some time still, hence it can be used. The best bet is to find a suitable tag or user who listens to very specific music.