Jesse Cook and Pierre Bensusan are two classical guitarists that I listen to whenever I need to relax or chill out. Very low key stuff.
If that's not your style, I'm also a fan of Paul Desmond, the smooth jazz saxophonist. It's very 50's style sounding, but it does the trick for me.

When I want to listen to music that's not too heavy on the mind, I listen to Clazziquai.

I know they've both been cited already, but Sigur Ros and M83 are absolute musts for this kind of music. You should particularly look into Sigur Ros's () album (that's the album's names... two paranthesis!). M83 is great too if you're intro shoegazing, their new album is a solid effort.

You could also look into Explosions in the Sky. They're an instrumental post-rock band centered around electric guitars and drums.

Another obvious one would be Pink Floyd, some of their stuff is pretty soothing (Wish you were here album, for example)