I'm looking for some good ambient music, so I'm wondering what you guys recommend. Some of the songs in the Metroid games (specifically the Prime series) and R-Type games (Delta, Final, and Tactics) are right along the lines of what I'm interested in because of their serious tone and slow tempo.

I don't mind what genre the game is from, so feel free to mention any game you can think of. :)

I think the "Abandoner" and "No Reality" soundtracks might be worth a spin.

You could also try the Parasite Eve 2 soundtrack, the Phantasy Star Online soundtracks, and the Baroque soundtrack.

Ambient music? Well there's Silent Hill and Resident Evil for starters, if you're interested in creepy ambiance music, although the first SH is the only one that has a bunch of in-game ambiance tracks, the others are mostly non-game tracks and music played during cutscenes and such.
Also try the soundtrack of Dead Space, the main theme is scary.

The Super Metroid OST is also a good option, but some of the tracks are incomplete (like the music played in the reddish area of Brinstar) and I read other tracks end up abruptly.

Doom 64. Xenogears might have a few tracks too.

Fallout 1 and 2 are my recommendation to you. Dark Ambient at it's best.

I'd suggest taking the Manhunt soundtrack for a listen or at best a gamerip which would contain all those extra ambient tracks not featured on an official soundtrack.

How about Mass Effect? the ost is on youtube if you want to check it out. I thought about this one when I read ambient --
http://www.youtube.com/watch?v=ZkbMf8QUrBI

Thanks for all the recommendations. Some of the soundtracks have a really distinct style that I enjoy very much (like kt2's music in Abandoner and No Reality) while others are quite haunting and evil-sounding (Silent Hill, Fallout, Manhunt, etc.). I'd like to add a few titles to this topic for those who are interested in ambient/moody/evil/weird music. Check out:

American McGee's Alice
Call of Cthulhu: Dark Corners of the Earth
Ecco the Dolphin (Sega CD version)
Fatal Frame
Haunting Ground
Parasite Eve
Siren

The Guild Wars: Factions soundtrack is amazingly ambient 95% of the time. I love it.

I wouldn't expect less from Jeremy Soule.

fatal frame 1 2 and 3.
you want creepy/scary/heartattacking ambiance? thats the only answers.

Best creepy-ambient soundtrack out there, in my opinion, is S.T.A.L.K.E.R. - Shadows of Chernobyl. Close your eyes while listening to that, and you're in a totally different place.

For ambient, off the top of my head, some good ones would be: Resident Evil 2, Parasite Eve, Final Fantasy VII and VIII have a few ambient tracks, Max PAyne has a few, Condemned, Silent Hill, Manhunt is all ambient. Pretty much anything stealth or horror related might apply.

Perfect Dark, Perfect Dark, and... Perfect Dark :P Plus whatever everyone else has said.

Oh, and Goldeneye 007 as well.

I'm mostly on ambience of horror. but...

I would recommed:

Eternal darkness sanitys requiem
Ecco songs of time
Phantasy star online (the part 1 of each level)
American mcgees alice
Resident evil soundtracks....