Some years back, I was attending Gen Con (the national convention for roleplaying games/card games, put on by Wizards of the Coast, who make Dungeons and Dragons). Amongst the many awesome items in the convention hall, there was an official soundtrack for D&D by a group called Midnight Syndicate. It was a combination of inspiring music, creepy music, rousing music, atmospheric music, and sound effect tracks (like muffled voices behind a door, or the roar of a dragon and his fiery breath). Overall, a completely awesome thing.
I looked them up on the internet when I got back home and purchased damn near everything else of theirs I could as it came out. They mostly did scary, atmospheric mood music with occasional sound effects and atmosphere blended in. Their CDs often created a story with the music, taking you on a sonic journey. Realm of Shadows had you exploring the ruins of a town in the countryside that had been overrun by some unseen malevolent force. Gates of Delirium took you inside a haunted insane asylum. The 13th Hour brought you inside a haunted house rising up from the mist. Others were in a similar vein, and all brought something awesome to the table.
I often find music with words in it to be distracting when I write, but the non-vocal mood music of Midnight Syndicate fits the bill perfectly.
Another group is in the same kind of niche: Nox Arcana. (And as a matter of fact, one of the fellows who was in Midnight Syndicate is now in Nox Arcana.) They have a broader range of music, not only having the haunted house, haunted asylum, vampires, and fantasy CDs, but also CDs based on pirates, fairy tales, carnivals, the works of H.P. Lovecraft and Edgar Allen Poe, and even the wonder of winter (all with at least a slightly eerie bent).
A side project of theirs, with the company Buzz Works, is about zombies, and one of their vocalists (for their infrequent but lovely vocal songs) has her own CD, called Blood of Angels (which is gothic poetry brought to song).
And if you want something really creepy and eerie, you can't get much better than AKLO, the music of the Cthulhu Mythos. Their two CDs, Unnamable and Beyond Madness, have music that sounds like it's from a parallel universe, and invokes great unease in everyone I've ever had listen to it. I use it in gaming sessions when I want to tell people without words that they're really in trouble now. They even have a more modern-sounding CD from their side project, Tillinghast Laboratories, called Sensitive Document. That one is techno that sounds like it was written by something not quite of this world.
All of these various CDs and their artists have special places in my music library, and have provided hours of enjoyment both for inspirational writing and atmospheric gaming.