- Thoughts of the Long Now - This is a free compilation of drone-style tracks. It's about 2.5 hours of music, some pieces more applicable than others. I'll pimp Emdot_Ambient's piece Realizations of Absolutism Vol. I as my favorite (and I'm not just saying that because he's one of my best friends).
- Ambient 1: Music for Airports - One of Brian Eno's early ambient works, this album is a defining piece in the ambient genre. The liner notes sum up the intent of ambient music and hint at why it might be useful in a gaming environment.
- Thursday Afternoon - Another work by Eno, this piece was originally a soundtrack to a video.
- Numena/Geometry - A collection of works by Robert Rich. A bit more structured than the pieces by Eno, still quite useful.
- Quiet Music - Steve Roach is very much a minimalist. Quiet Music fits right in Eno's concept of ambient works.
- Dark Wood - Classic cello in a very melancholy arrangement. Some of Darling's other works are more intrusive, but this album is quite fitting as game background.
- Stone Tower - This is a fairly dark work, a bit noisier than some of the other pieces mentioned. It works quite well in tense settings, lending a bit of a horrific feel to the scene.
I hope this inspires you to give music a try at your sessions.