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:

• 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.