FSOL: Lifeforms, Environments II, and pretty much everything really
The Orb: Adventures and Terrarum are the only two I really class as ambient
Biosphere: Most of it, but particularly Substrata and Cirque
Spacetime Continuum: Sea Biscuit
Aphex: Both Ambient Works, particularly II
Eno: Ambient 1 and 4
Global Communication: 76:14
KLF: Chill Out
Gas: Zauberberg (never really dug the other three)
Woob: 1194
And a whole heap of other obvious stuff
Also,
Apollyon: Veror Aesik
Encomiast: Espera
Off-Land: Commute
Wintermute: Widescreen
Carbon Based Lifeforms - Hydroponic Garden
And anything of Underworld's that's mellow and chilled, which is a fair bit.