this might not be what you're looking for, but my ultimate chill out song is "children" by robert miles. if played at a lower volume, it really gets me relaxed and just chilled.
check out some post rock. i am a big fan of sigur ros, mogwai, eluvium, jesu, explosions in the sky, even a little bit of the newer sonic youth stuff(i love their older stuff too, but its not quite as mellow).