Comment by krylon
Sleep by Max Richter is great (and very long)
Sunset Mission by Bohren & Der Club of Gore is very very sleepy Jazz (they have released more albums, but this one is my favorite by a wide margin)
Long Ambients 1 & 2 by Moby - he was kind enough to make them available for download free of charge, too
Under Wires and Search Lights by Marconi Union
In A Silent Way by Miles Davis
Pretty much anything by Sigur Rós. It's not strictly speaking instrumental, but the lyrics are Icelandic, which I don't speak, so it's close enough
Cocteau Twins recorded many very ambient-ish albums. Not instrumental, but the "lyrics" are mostly glossolalia, so not distracting (at least for me).
No particular order:
Max Richter, John Cage, Tangerine Dreams, Klaus Schulze, Gavin Bryars, Richard Chartier, Asmus Tietchens, Tomaga, Boards of Canada, Stars of the Lid, William Basiniki, Joanna Brouk, Pauline Oliveros ...
I do way too much coding...