The music we play by

[Ghost]

Any 'Dead Can Dance'.  Ambient music of all kinds, particularly some Brian Eno.  Nusrat Fateh Ali Khan for eastern themed games.  Classical music in general.    

Seconded.

I find that most dance/trance tracks contain too many vocals/unfitting beats to suit most rpgs. Really, when you're fighting a Beholder, the last thing you want to hear is someone singing about dancefloors.

[Ishmayl-Retired]

I used to plan out big soundtracks with various movie, video game, and TV show scores, but nowadays, since we play mostly online, I just don't have time or inspiration.  Here are some examples of things I used:

Batman Score (Danny Elfmann)

Last of the Mohicans Score (Randy Edelman)

Star Trek VI Score (Clif Eidelman)

Buffy and Angel TV Show Scores (Christophe Beck, Robert Duncan, Robert Kral, Others I don't know the name to)

Various Classical and Romantic Symphonies and Operas (Giacomo Puccini, Gustav Mahler, Franz Liszt, Sergei Rachmaninoff, Mily Balakirev, Modest Mussorgsky, Nikolai Medtner, and Pyotr Tchaikovsky) - As you can see, I tend to like the darker music more than anything else.

If I were to still use music for sessions, I would probably use:

Serenity Movie Score

Firefly TV Show Score

Lord of the Rings Movie Score

Lost TV Show Score

X-Men 2 Movie Score

Spiderman 2 Movie Score

Batman Begins Movie Score

Punisher Movie Score was pretty good and had some nice parts

Harry Potter Movie Scores (especially PoA)

That's about it for now... if I think of more, I'll add.