Whether or not I use music depends upon the group and the genre of RPG. Some groups like music, others want silence, others want the "big game" on in the background. Still others have zero preference.
A sampling of what I have used includes:
Singing Bowl gamelons
Anything by Kodo (a modern take on Japanese Tycho drumming)
The western jazz/traditional arabic music fusion of Rabih Abou Khalil
Stuff by Rick Wakeman (keyboardist for Yes), Phillip Glass, Vangelis (esp. "Soil Festivities" and his stuff with Aphrodite's Child), Andreas Vollenvieder.
Assorted hard rock/metal albums
Stuff by Tangerine Dream- esp. the early psychedelic stuff like "Atem"
Instrumental work by Glen Branca, Bill Laswell (also see Material, Death Cube K, and more), Buckethead, Jonas Hellborg and others.
Soundtracks to "Conan", "LotR", "1984", "Legend", "Blade Runner", "2001: A Space Odyssey". "The Last Temptation of Christ", "Koyaanisqatsi", "Clan of the Cave Bear", "The Hunted" and others.
Classical guitar by the Romero Brothers
Classical music like Holsts' "The Planets" or Rimsy-Korsikov's "Firebird Suite"