There are two types of ambient music, the natural white noise of nature and manmade audio. In terms of nature, there are some great recordings of rivers rushing, fire crackling, thunderstorms rolling in, and birds chirping in rain forests. Now, remember, nature's ambient music can't have any vocals or instruments or manmade noise, in my opinion.
After a search that took me practically to Brazil and back, I found my copy of Rio Chimehuin: Three Audio Perspectives from Garuda Records by Ed Herrmann (1998).
Some great rainforest and birds music is Remote Thunderstorm & Bird Songs (Orage lointain et chants d'oiseaux) by John Grout on his Relaxing Sounds of Nature album [it's on Itunes too]. Another awesome one is L'aube dans la forêt (Costa Rica) by Nature Sounds on their album Birds of Paradise (Oiseaux de paradis) [also on Itunes].
For the other kind, I like Pan American, especially their The River Made No Sound. I started a collection of their cds when I was a kid. Sometimes the pieces remind me of Mondrian's early classic pieces. In another way, with more vocals, I find The Skygreen Leopards to have a lot of the same qualities--just in another genre of music, more toward folk.