ALRIGHT! Here I'll open up a can of worms.
I've always been fond of playing background music for games, and I'll often look to soundtracks for that.
My favorite musician is Gary Numan (most famous for his 1979 hit
Cars, but he's a prolific worker who continues to make new music, and released what may be his best album ever in 2014- and he's working on a new album to release mid next year). He has done two score albums, which I often put on to get in the mood.
The first was done in 1992 called
Human, which has a late 80s synth dystopia sound. I grab it for gritty sci-fi type of games, cyberpunk or weird space opera stuff. It could also work for some horror games.
He also released another score in 2014, called
From Inside. It's got a more horror sound, maybe something like Silent Hill horror, although parts of it make Me think of Halo.
I'm more inclined to space fantasy stuff, so the
Tron Legacy soundtrack is also frequently referenced. A lesser known gem is the soundtrack for the underappreciated cartoon prequel/tie-in
Tron Uprising, which has a bit more variance than Legacy.
Going more esoteric, the soundtrack for the 2002 live action
Casshern is a powerful and haunting score, with a fair amount of rocking action music and beautiful bits. I also really like the movie, but apparently I have bad taste for that... Unfortunately, it seems Youtube has taken down various videos of the soundtrack, making it harder to find some pieces.
For a more fantasy feel, I've recently taken a liking to the soundtrack for the game
Drakengard 3, although that might sound a bit too repetitive as it uses a one woman wail sound. Apparently it's spiritual sister game Nier also has an excellent soundtrack, and the upcoming sequel Nier Automata looks to follow suite.
I also love the soundtracks for both
Vampire Hunter D and
Vampire Hunter D: Bloodlust. I slightly prefer the older one, both the movie and soundtrack, but I think more people would gravitate towards the newer one.
Going completely off the rails, there is also a soundtrack made for the graphic novel
Purgatori: The Vampire's Myth. This one is a bit harder to explain, since youtube apparently has never heard of it.

It mixes classical rock with middle eastern sounds and Hellish ambiance and all sorts of stuff. I think it's pretty great for dungeon crawling or demon realm adventures.
Okay, I think that's about enough yammering from Me.