Terry Riley's A Rainbow In Curved Air was massively influential on prog - do a search and you'll find some very informative posts by Dick Heath on this topic, with a few rather facetious ones from me. There's a great version of In C by New York percussion ensemble Bang On A Can, who also did a great re interpretation of Music For Airports.
Of approximately the same vintage as Terry Riley is Steve Reich, whose work is a lot more engaging than titles like Music for 18 Musicians might suggest. He did an interesting collaboration with Pat Metheney, Different Trains.
Probably the most famous composer in this style is Philip Glass - the best place to start is probably his soundtrack to Koyaanisqaatsi. He's also recorded symphonic treatments of Bowie's Low and Heroes albums, which are a lot better than most rock/classical crossovers