The folks over at Gnome Stew have an article about using Pandora for background music in your games. Unfortunately, Pandora itself isn’t accessible from outside of the US, but the article still has a few solid music selections, to which I would add the following artists: Two Steps From Hell, Mogwai, Laura, Explosions in the Sky, and E.S. Posthumus. As you can see from the suggestions, post rock is a good genre for background music as it generally has no lyrics and can be effective for evoking particular emotions or feelings. I’d also suggest Grooveshark as an alternative to Pandora. If you register, you can have it build dynamic playlists from what you like.
Check out the article Gnome Stew