Alright, well...
Frank Zappa was best known for his rock music, but he originally set out to be a modern composer. Towards the end of his career he began recording music with more classical-like ensembles. For instance, his work with the London Symphony Orchestra, (Check out Frank Zappa, LSO Vol. 1). The Ensemble Modern (Check out The Yellow Shark, there are videos of it on youtube.... Dog/Meat, also Then Ensemble Modern plays Frank Zappa: Gregarry Peccary and other Persuasions) and his work with Pierre Boulez's Ensemble Intercontemporian (The Perfect Stranger). All of his studio albums are great too. Particularly Roxy and Elsewhere, Hot Rats, One Size Fits All, there are sooo many.
Toru Takemitsu is a japanese composer (20th century) who was influenced by Debussy and Messiaen. His work is really interesting because he blends elements of Eastern and Western culture, philosophy, and musical ideas. Check out
Conlon Nancarrow is this crazy guy who was from Arkansas, but involved with communism and all this crazy political stuff and ended up an exile in Mexico. He was discovered by Ligeti, I believe. He wrote a lot of music for player piano that is unplayable by humans. It is really, really wild stuff. I also like his string quartets 1 and 3.
Oliver Knussen might also be worth mentioning but I havent really heard that much of his stuff. He was friend and interpreter of Takemitsu and his music.