Wow... No one has walked the hallowed halls of this thread for some time!
Bringing things into the current decade, I'd like to add that some of my favorite gaming background and mood music is sourced from games like Elder Scrolls Morrowind, Oblivion, Dragon's Age and The Witcher. LOTR (as already mentioned) is a great go-to, though I like to use the less iconic tracks from the trilogy that aren't as easily recognized. It helps lend more to immersion and less to meta-playing.
