2/1, one of four pieces on Brian Eno's groundbreaking Abient 1/Music for Airports.
Stria, by John Chowning, explores the sound generating capabilities of FM synthesis to create overtones at unusual ratios based on the Golden Ratio found in nature (1:1.618),
Six Fantasies on a Poem by Thomas Campion, by Paul Lansky, uses computer generated voice (similar to a vocoder) to "sing" the text with a rich and complex harmony.
Snowball, appeared on Moby's album, Hotel. Disc 1 contains normal songs, but the second disc contains only ambient pieces derived from the synth work on those songs.
Moby's Overlands is another example from the same album.