Man, I have so many possible recommendations.. need to think and whittle down. I'll edit my post regularly, adding more and more as they come to mind.
First, I'll pretty much second all the recommendations everyone has already listed. oosoo's recommendations are definitely spot-on, except for the Richard Devine recommendation. I'm not terribly fond of Devine, and even if I were, I wouldn't consider him "dark" at all. But, hey, different strokes.
jhawk's d&b recommendations are also good. Panacea's "Low Profile Darkness" is an absolute classic and was
way ahead of its time. (Be warned: new Panacea is ENTIRELY different from Panacea of old.)
If you want dark ambient, I'd recommend starting with the
LOKI Foundation label. Inade especially, as well as the aforementioned Herbst9 recommendation.
Malignant Records is a great, cheap American mailorder that stocks the aforementioned.
For the epitome of dark and doomy, Sweden's Brighter Death Now is where it's at. Keep in mind this stuff probably wouldn't be considered "music" by most. It's loud, abrasive, noisy, and usually devoid of things like rhythm and melody... it's all atmosphere. But if you can get into it, it does its job very well.
Okay, more later...
- Chris