Just my own 2 cents. Two Steps From Hell is the perfect ambient gaming music for almost any setting! They sound like a heavy metal name, but they are not at all! It's short compositional tunes without lyrics and by Jove, they are amazing!
Also, check out Evil Cult (a fun little browser game) - at one time, the ambient music in it was a free download (bugger if I can find the webpage again, but I have the songs) and is great for creepy - especially illuminati style.
In general, I avoid music with lyrics as it can be overly distracting.
EDIT: The Evil Cult music can be downloaded (legally) here:
http://code.google.com/p/cult/wiki/About