There is a lot of research suggesting music is capable of focusing the mind and relaxing the body. Everyone has their own preferences but I like listening to the following on my iTunes. (Click on the iTunes link to listen to previews)
Board of Canada 
Tracks: Constants Are Changing, Sherbert Head, Slow This Bird Down etc
Album: The Campfire Headphase
Reason: Very chilled out, can play in background without noticing- Portishead
Tracks: Strangers, Sour Times, Pedestral
Album: Dummy
Reason: Like the big room feel to it and the trip hop genre in general - Cinematic Orchestra
Tracks: The awakening of a woman
Album: Cinematic Orchestra
Reason: So cinematic, plays in the background without realising - Moby
Tracks: Down Slow, Run On, The Sky Is Broken
Album: Play 
Elbow
Tracks: I’ve got your number, Buttons and Zips
Album: A cast of Thousands- Thievery Corporation
Tracks: Marching The Hate Machines, The Cosmic Game, A Gentle Dissolve
Album: The Cosmic Game
Obviously some people will look through the list and think awesome this guy knows his stuff, others will think who the hell are they? and some of you will head off and download some of the tracks and find that it works for you. Zen Habits also suggests a good tranquil music list.
If your a music person then post your favourite tracks for relaxing in the comments.