Sigur Ros’ album that’s all white and titled “( )”, and, appropriately enough, Brian Eno’s ambient album Thursday Afternoon – which was used in the movie 9 1/2 Weeks, playing softly in Kim Basinger’s apartment, IIRC. 
The Buddha-Bar series is great too.
Or the “new Nick Drake”, Iron & Wine.
Love Buddha-Bar. I think 6 is the one I have, it’s white and Buddha is gold.
My favorite CD to listen to when I’m stressed is George Winston’s December. I like it all, but specifically I like his variations on the canon by Johan Pachebel. Link
I’ll go along with Eno, Deep Forest, Shpongle, Air, Buddha Bar & Stereolab. I’ll add:
Zero 7
Aphex Twin - Selected Ambient Works Vol. II
Ulrich Schnauss
Red Jerry - Deep & Chilled Euphoria
Chill Trance Essentials series
The Orb - “Little Fluffy Clouds” & others
Moby - “The Sky Is Broken”, “When It’s Cold I’d Like To Die”, “The Rain Falls & The Sky Shudders”, “God Moving Over The Face Of The Waters”, “Hymn”, “Guitar, Flute & Strings”…
Maybe not what the OP is looking for, but I tend to like listening to Massive Attack’s “Protection” when I’m looking for something mellow that still has a beat to it. Great album.
That’s a really good suggestion, his Autumn album is extremely relaxing as well. Perfect thing to go with a good book, a warm couch, and a rainy winter day.
IIRC, I have VII. Bright blue and gold. I’d have more of them, but Jebus they’re expensive.
A lovely, languid song, but it’s from Before and After Science, not Another Green World. And only what used to be Side 2 of BAAS (in the pre-digital days) is peaceful; the first half of the album being mostly pretty frenetic.
I’ll second **Massive Attack’s ** Protection - great, great CD - other Bristol / trip-hop bands include **Portishead ** (get “Dummy”) and **Morcheeba ** (first two albums are great).
Really surprised no one has mentioned **Miles Davis’ Kind of Blue ** - not only one of the greatest albums of all time, period, but incredibly mellow. Can be perfect for a Sunday morning, or a very romantic evening.
My obscure recommendaton is Tabula Rasa by Arvo Part, an Estonian composer. I picked it up when he and that work were profiled in The New Yorker and there was a lengthy discussion of how terminally-ill patients found this piece calming, moving, but ultimately uplifting. It is all that an more - there is so much open space in the piece, but it is amazingly subtle and moving. (on a side note, I had it on once when my young son and I were building a model together; I asked him what he thought of the music and he replied “it’s cool - it sounds like Queen Amidala is entering a big room” so I guess that means it must be good!)…
You second me, I’ll second you. Kind of Blue is one of my all time favorites, but I would never have thought of recommending jazz because jazz seems to annoy a lot of my friends.
Some suggestions off the top of my head…
Rounds by Fourtet
Pygmalion by Slowdive
Tired Sounds of… by Stars of the Lid
Young Prayer by Panda Bear
Ambient 2 by Brian Eno and Harold Budd
Farenheit Fair Enough by Telefon Tel Aviv