There's so many different kinds of music that relax me... even ones that would normally pump me up. It depends on my mood. Strangely, I find punk rock to be very soothing... I can fall asleep (and I have) in the corner of the local venue while the band rocks on, if I'm in the mood to.

J-pop in general is very soothing... particularily L'Arc~en~Ciel and Bonnie Pink. Classic rock like The Beatles, The Who, Zeppelin, and The Doors really mellow me out, especially when there's some inscense and nice company. Oh and the theme song to Meteor Garden (the Chinese live action version of Hana Yori Dango) is my current relaxing song. It's so beautiful.