Background music is a big part of running games for me. With a small group, it's nice to have something to fill in the background. And occasionally, I will put on a particular piece to set the mood. I prefer instrumentals almost exclusively.
With Emily's character Lennon being a huge Beatles fan, she tends to bring music to play with a lot of Beatles and John Lennon, with some Firefly/Serenity music mixed in, along with some asian themed music like the soundtrack for Memoirs of a Geisha as well as some western themed music.

My play list for Serenity games runs to movie and video game soundtracks, with some other stuff thrown in:

Firefly TV series
Serenity movie
Halo 2 video game, volume 2
Halo 3 video game, volumes 1 and 2
Hero movie
Cowboy Bebop anime series
Assorted songs from Blue Planet: Seas of Life TV documentary series
Memoirs of a Geisha

Fan Music
Bedlam Bards On the Drift album (instrumentals only)
Persephone Pickers Sounds of the Verse instrumental album

"The Scientific Method" by Eric Serra Bandidas soundtrack
"Takarasaka", "Lil' Medley", "We Hide and Seek" by Jerry Douglas
"Slamming" by Arlen Roth
"Rosc Catha Na Mumhain/Arkansas Traveller/The Wild Irishman" by the Chieftains
Space: Above and Beyond TV series theme song
Assorted Beatles songs, mostly from The Yellow Submarine movie soundtrack and The Beatles 1967-1970 album

I can also add the soundtracks for Crouching Tiger, Hidden Dragon, The Last Samurai and The Last Airbender if I need more asian themed music.

Update: I LOVE Blues Saraceno's "Blood will Spill", "Gravedigger" and "Strident Missile" are great too. "Blood will Spill" sounds like a religious Browncoat praying at the Battle of Serenity Valley; he asks for forgiveness for his deeds and mercy for his fallen opponents.

I am currently playing Rebel Galaxy which has music by Blues Saraceno and others:
Blues Saraceno – Pumpin’ Irony inst.
Blues Saraceno – Evil Ways
Blues Saraceno – Smokin’ Fire
Blues Saraceno – Killin’ Floor
Blues Saraceno – Bad Man
Blues Saraceno – 7th Born Son
Kenneth Sorenson – God Damn Better
Kenneth Sorenson – S’Alright
Nick Nolan – Hell on Wheels inst.
Dolph Taylor/ Bart Hendrickson – Quaker Bait
Sam Wale – Screen Door Slam
Andy Hall/ Clarence Buzz Chestnut – Railroad Bible
The Blue News – Old River
The Blue News – Promiseland
The Blue News – Same Old Story
The Blue News – Stay Gone for Good
Big Blue Barry – Bayou At Night
Big Blue Barry – Not Afraid To Fight
C-Leb & the Kettle Black – Heavy Hands
Flytrap – Killing Love
Nicholas Tremulis – Money Jungle
Pat Andres – Dobro Slider Blues
Phil Symonds – Lead Foot
Abbas Premjee -Blue Billy
Abbas Premjee – Moonshine is Fine inst.
The Loomers – Ready to Rip