Everyone should add here beatifull relaxation songs like:
Moby - Porcelain
Sarah Brightman - Time To Say Goodbye.
Richard Clayderman - Ballade Pour Adeline
Secret Garden - Nocturne
Vanessa Mae - Widescreen
Julee Cruise - Falling
Gerally I use unhappy songs to relax. Is good to reflit my thinks too
Here that some:
I´m With You - Avril Lavigne
Look What You´ve Done - Jet
Denial Revisited - Offspring
Shape From My Heart - Sting
I Wish it Would Rain Down - Phil Collins
Welcome to My Life - Simple Plan
The Reason - Hoobastank
If you want to relax listen to Buckethead's Electric tears cd, it is unline ANYTHING else he's every done and will put you in the most relaxed moods nearly instantly.
Listen to Sigur Ros, Explosions in the Sky or Mum if you want to relax. My highest recommendation goes to an album called Takk by Sigur Ros.
I relax to john mayer and some pearl jam 
Opeth, songs like "Harvest" "A fair Judgment" "Death whispered a lullaby" and "windowpane"
Some Old school Metallica, like "Sanatirum" "The call of Kutlu" "The Unforgive (even though its not old)" and more of that type.
RHCP, like "Otherside"
and thats what I'm playing right know, bout to go to bed 
