Hmm.. I listen to a lot of different music, but when I really want to chill out, I'd have to say Pink Floyd is my go-to band. My wife has almost every album they've ever released.
We listen to a good bit of different music and we always have but when I want to rest or just relax in the hammock out back I have a little radio. I can listen to a few stations on it when the weather is good but my favorite is the jazz station (light jazz).
Blues and old school country can make me feel very relaxed. In the summertime, I love to come home after a hard day at work, turn on the stereo, and kick back on the deck with a beer listening to some Willie, Waylon, Don Williams, etc.
I like to listen to smooth jazz when I want to relax. My wife likes to listen to new age music and classical when she wants to relax. If I really want to relax and be lazy at the same time, however, I will usually just veg out and watch some television.
Really depends on the mood. Pink Floyd like mentioned is great. Sometimes I may play some classical. Lately it has been more Celtic Punk/Rock like The Tossers.