I recently got to run two sessions of Call of Cthulhu for my group, and when I was prepping for the game I decided it was time to freshen up my horror background music playlist. I normally use an “applicable to most RPGs” playlist (Sinister, part of my three-playlist system  for BGM) that draws from all sorts of soundtracks, but a little web research suggested a new tack that sounded fun: dark ambient music that fades into the background, its atmosphere of tension pervading the game.
My poking around led me to acquire several new albums, including some excellent legally-free ones, and I liked how they worked as BGM. I’ll be using them — and expanding my catalog — for future horror games. (This Yog-Sothoth thread  was a goldmine for recommendations.)
I made two new playlists, Ambient and Tension; I used the former most of the time, and the latter when things were getting tense. During action scenes, I focused on describing things in an action-y way and left Tension playing in the background rather than switching to an action playlist.
For the lists below, I broke the albums down by free/not free and noted whether I used them in my sessions or not. The ones I sampled and didn’t acquire all sounded good (or I wouldn’t mention them here), but weren’t right for my session (or, in some cases, I just plain ran out of time to sort them into playlists, and so didn’t download them). They might be perfect for your game.
Verbum Mentis: Refrigerium 
Verbum Mentis was a great find — this is music that makes me think of spelunking, trapped deep under a million tons of earth, wondering what’s down there with me. (I’ve actually been spelunking once, and my memories of that trip come back when I listen to this.) So good. I used this album for my game.
Verbum Mentis: Past appearances I 
For a musical connoisseur, which I’m not, it’s probably easy to tell which Verbum Mentis album is which…but I can’t. And that’s fine! They’re obviosuly not identical, and they’re all good. I used this one as well.
Verbum Mentis: Past appearances II 
My third Verbum, also great, and also used on game night.
Olivier: Obscure 
I believe this is the soundtrack to a video game. It wasn’t right for my game, but I liked the samples and could see using it in another context.
Igor Stravinsky: Symphony of Psalms 
I try to avoid vocals in my BGM, save for the occasional track here and there, so this choral album isn’t quite my cup of tea for gaming music. But creepy? Holy shit is it creepy. It also sounds old, like it’s being played on a wax cylinder or something — really perfect for the right kind of Call of Cthulhu session.
Atum: Agorafobia 
I ran out of time, or I would have picked this up; it’s really solid. It’s sold on a “name your price” basis with a minimum price of $0.
ZewCthulhu.pl: Shub-Niggurath 
Like Agorafobia, I ran out of time to categorize this one or I would have bought it. (And like the Atum album, it’s also available for $0 and up.)
Reasonably priced albums
Musica Cthulhiana: Fragment 
Musica Cthulhiana — surpise! — makes music with Call of Cthulhu in mind. Their stuff is broadly applicable to any kind of moody horror, though, and it’s fantastic. This was my favorite acquisition.
Musica Cthulhiana: The Fourth 
Almost as good as Fragments, and distinct enough to make it worth owning them both. I used and enjoyed this one — and like Fragments, I sometimes listen to it while I work, which makes for some creepy writing music.
Toru Takemitsu: Flock Descends Into The Pentagonal Garden 
It might not look or sound like horror, but give it a shot and you’ll see why it works. I used it in my session, and it’s quiet (sometimes a bit too quiet for BGM), subtle, and creepy in a way most BGM isn’t.
Lustmord: Carbon / Core 
I’ve seen Lustmord recommended for BGM many times, and after listening to samples I can see why. Based on the albums I checked out, this one seemed like the easiest fit for a horror game.
Pelgrane Press: Eternal Lies Suite 
If I’d run across this earlier, I’d have bought it first — it’s Pelgrane’s soundtrack for their Trail of Cthulhu RPG, and holy crap does it sound good based on the samples. This is on my shortlist the next time I buy any horror music.
I wish there were more publishers out there commissioning music for their games, because it’s an awesome tool to have on hand.
Allicorn: In Remote Places 
This would make a great soundtrack for Beyond the Mountains of Madness — or any game with a wintery setting, for that matter. It makes me think of cold places, wintery nights, and what could be lurking in the snow.
Glenn Danzig: Black Aria II (NSFW cover) 
This sounded like an out-of-left-field choice, but listen to a couple of samples before dismissing it. I’m not sure what sort of game I’d run with it, but it’s got potential.
Now that I know this approach works, I’m definitely in the market for recommendations — and I bet other GMs would love to hear about your favorite dark ambient albums, as well. If you’ve got some good ones to share, let us know in the comments!