Bioshock 2 - Score (Sounds from the Lighthouse)
Quentin Tarantino's The Hateful Eight (Score)
Thomas Bergersen - Illusions
Maleficent - OST James Newton Howard
The Brothers Grimm OST
Silent Hill - (There is an 8 disk, toss out the songs that don't fit D&D and your good.)

I've been thinking of music for a Vampire The Masquerade game and in terms of source material, Mafia II's OST (both the licensed songs and the score) and the Urotsukidoji OST by Masamichi Amano are excellent for my BESM Vampire The Masquerade game set in Wise, Virginia circa 1993-2001. In terms of genres, I'd consider classic rock, oldies, and bluegrass as well.

I like Tangerine Dream quite a bit, Vangelis, and Phaedra as well, for gaming. Other Metal that I listen to that always reminds me of D&D included Molly Hatchett, Iron Maiden, Whitesnake, Black Sabbath, Dokken, Nazareth, Jethro Tull, King Crimson, Uriah Heep, Blue Oyster Cult, Boston, Emerson, Lake & Palmer, and of course, Rush. Always thought By-Tor and the Snow Dog from the Fly By Night album was inspired by the Silmarillion and was about the Elven hound Huan and his epic fight with Morgoth when he swallowed the Silmarils and ran away stripping Morgoth of his power. There was also of course, Rivendell. Led Zeppelin of course, not to be outdone also did up the LoTR based "Ramble On", about Frodo.

Basil Poledouris - The Atlantean Sword (you guys probably know him from the Conan Soundtrack)
www.youtube.com/watch?list=PL5A276DC5DDA2B4D9&v=XtaYmsOi908&feature=player_detailpage#t=11

Also like Jo Blankenberg - Knights of Palmyra
www.youtube.com/watch?v=Qii3CbAO3VE&list=PL04542A3218E38D14&index=17

There's lots of interesting Norse folk music and metal that I find inspirational for D&D, Just some of these bands and performers include;

Gjallerhorn - Suvetar
https://www.youtube.com/watch?feature=player_embedded&v=OD_TIrX7zu0

Gjallerhorn playing Ragnarok
//www.youtube.com/watch?v=OfndOsKEOxs

Vasen,
//www.youtube.com/watch?v=tWorsJwzycw&list=PLk11FImlQMJnRrRi1R5qKxWw4a4ja__0t

Aly Bain, especially Full Rigged Ship
//www.youtube.com/watch?v=GdUikhLuKtU

Garmana. Garmarna has an awesome album based on religious songs written by the nun Hildegard Von Bingen, titled of course, Hildegard Von Bingen

Garmarna - O Vis Aeternitatis
https://www.youtube.com/watch?feature=player_embedded&v=pO85Azdf2sA


Garmarna - Hildegard Von Bingen
www.youtube.com/watch?v=DBWJhq5LcII

Aly Moller
www.youtube.com/watch?v=KqQ0WaWrVt0

Njordfoto
www.youtube.com/watch?v=YypKod0wBdc&list=PL5A276DC5DDA2B4D9

Adrian Von Zeigler - Gliepnir
www.youtube.com/watch?list=PL5A276DC5DDA2B4D9&v=gdb9RDZp7Go&feature=player_detailpage

Annbjorg Lien - Loki
www.youtube.com/watch?v=GquiFbPswT4

Annbjorg Lien - Galadriel
www.youtube.com/watch?v=FTHaT36xJFU

Feather and Skull
www.youtube.com/watch?list=PL5A276DC5DDA2B4D9&v=9RuNdb1Dt0o&feature=player_detailpage#t=6

Skilfingr
www.youtube.com/watch?list=PL5A276DC5DDA2B4D9&v=LVH91f3GLmM&feature=player_detailpage#t=3

Tyr - Hammer of Thor
www.youtube.com/watch?list=PL5A276DC5DDA2B4D9&v=yH-iX-TTJcg&feature=player_detailpage#t=5

Cry of the Celts
www.youtube.com/watch?v=DlHmGy4Aq_g&feature=player_detailpage&list=PL5A276DC5DDA2B4D9

Tuatha De Danann - Return of the Elven World
www.youtube.com/watch?v=DlHmGy4Aq_g&feature=player_detailpage&list=PL5A276DC5DDA2B4D9

Orda Funebris - A Witch's Song
www.youtube.com/watch?v=lePjSDl8eN8&index=83&list=PL5A276DC5DDA2B4D9

Adrian Von Zeigler - Wolf Blood
www.youtube.com/watch?v=06H_6oI4EK4&index=85&list=PL5A276DC5DDA2B4D9

Adrian Von Ziegler - Sons of the Allfather
www.youtube.com/watch?v=YKLJsxKGiGA&feature=player_detailpage&list=PL5A276DC5DDA2B4D9

Wardruna - Helvegen (The Way to Hel)
www.youtube.com/watch?v=z0PvZGVPiJU&index=90&list=PL5A276DC5DDA2B4D9

Wardruna - Fehu (A Viking War Song)
www.youtube.com/watch?v=Le4B9T12zbs

Hedningarna - Metsän Tyttö (Forest Maiden)
www.youtube.com/watch?v=RfipsKs8Qao

Hedningarna Playlist
www.youtube.com/watch?v=RfipsKs8Qao&list=RDRfipsKs8Qao&index=1

almost forgot Värttinä, a Finnish band.

Värttinä - Oi dai
www.youtube.com/watch?feature=player_detailpage&v=NFu7zGWdJa4&list=PLE2DBE68EF2D91C03#t=4

Värttinä
www.youtube.com/watch?v=l9ozxHOXf8I&list=PLE2DBE68EF2D91C03

Some inspiration Metal and Folk music from across the pond for you.

Well, my Wild West campaign has featured a bit of Johnny Cash.

Wow, that's wide range of stuff there.

For general unobtrusive background music I think I would go with maybe 1492 the Conquest of Paradise (Vangelis).

By the way, the computer game Vampire the Masquerade Redemption (the first VtM computer game I think) has excellent music.

Sci-fi: https://www.youtube.com/watch?v=goDd02TTxBA

Aphex Twin from his Selected Ambient Works II period.

Then there are whole lists: https://www.youtube.com/watch?v=UU4nEOYisCs&list=PLmGEbmwqAA4IYqCuH3bHzTVVtdpG6N4IJ

Sci-fi Ambient Music by Forgotten Gaming

I used to do this sort of thing a lot more.  Back in the day when I was running my Legion campaign, I had a big soundtrack, especially when the campaign got into its "space-disco" era.

Updated:   Version 1.4  (6/19/2016)

Adventuring:
Popul Vuh's soundtrack to Aguirre: Wrath of God,
Klaus Schulze's "Irrlicht,"
Earth's "Hex" & "Hibernaculum,"
Prokofiev's "Alexander Nevsky,"

Civilization:
Thai phin
Andean quena
Eastern European hurdy gurdy
Gamelan
Lou Harrison's "La Koro Sutro"

Lots depending on the vibe. I don't mind anachronism.

Yesterday, I started using http://www.tabletopaudio.com/ I'll be sticking to just that, as long as it lasts.

Version 1.5 (8/13/2016)

I often use music made by Nox Arcana or Midnight Syndicate as background music. Both ambient music made for roleplaying. Mostly for horror but some fantasy as well. I wouldn't listen to it normally, but with roleplaying it fades into the background.

Nox Arcana

Midnight Syndicate

D&D soundtrack

NieR OST, all of it. Drakengard 3 have some good OST, too.

Updated 1.6