off the top of my head I'd recommend boards of canada, lamb or portishead. Very chilled out, not too intrusive, and absolutely amazing. Some of portishead's newest album is a bit noisy, but pick up a copy of dummy. Get Lamb's first album and get whatever you can by the Boards of Canada, it's all good.
Oh yeah and get an album called the K&D sessions by Kruder Dorfmeister. Most chilled out album ever released.
good man - portishead are superb, not mad on their latest one but couple of good ones.
Aphex twin (bear with me...) did two ambient albums, but the second in my opinion is more ambient/noise than the first, which is more melodic.
Check out my fave track as an example - http://www.youtube.com/watch?v=w6CQRVODln0
(edit - and another - http://www.youtube.com/watch?v=ZVvjXJen ... re=related
If you are stuck I can "advise you of a location to preview it" :)
Some of it might seem a bit more suited to a heroin den but its a fine line (ambient that sooths and ambient that borders on haunting) so I conceed it may not be too popular generally
I also got this mix lately that I enjoy - Nihls Frahn, modern classical (its not too far from the normal genre but is some electronic stuff, just re-listening to it and theres some "glitch" ambient) http://cdn.official.fm/downloads/mp3s/9 ... 100328.mp3
Brian eno did some great ambient - Music for airports is around 6x 9-16 minute tracks, very minimal ambient.