Ahhhh, Harold Budd -- I live and breathe his stuff!!!
While he certainly isn't limited to spare, gently-treated solo piano music, his _best_ work, IMHumO, lands squarely in that particular sonic field. He's the crop-circle-alien-master of the genre, delicately balancing consonance and dissonance like a super-sedate (read: positively-meditative) Chopin. This is music that works well for both rest and activity; it can be part of the background ambience, but it rewards attentive listening, too!
The two Budd/Eno albums, "Ambient2: The Plateaux of Mirror" and "The Pearl," are absolute must-haves, in addition to his more recent works with John Foxx entitled "Drift Music" and "Translucence." He also has several other similar albums involving only himself (e.g., "La Bella Vista" and "Perhaps"), but I don't find myself listening to them as often; he seems to achieve perfection only in tandem!
"Our developers, who art in Toulouse, hallowed be thy physical-models.
Thy version 4 come, thy new instruments be done, in the computer as it is in the wood!"