And of course there's Aphex Twin's Selected Ambient Works 85-92, which should go without saying really. Again, you might not be able to work because it's such a good record. Also, it's not ambient in the sense of having no beats. If you've never heard it before you should probably just stop working right now and listen to it. Also he did a SAW vol. 2 album which is more beatless, but it's not as good.
You can't go wrong with Future Sound of London. It's a bit dystopian, but utterly amazing stuff.
The "minimalist" (he hates that word) work of composer Steve Reich is also very good for zoning out. All the other people in the modern "ambient" genre are ripping him off to some extent or another. I suggest starting with Electric Counterpoint.
EDIT - some more:
Hecq - 0000
Almost anything by The Orb, especially The Orb's Adventures Beyond the Ultraworld. Make sure you get the proper UK version not the cut down US release.
Portico Quartet by Portico Quartet
All the Boards of Canada stuff