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).