Wozzeck
Berg wrote this opera before the period when he used serialism in his works.  His teacher Schoenberg had not yet developed the twelve tone system.  The music of Wozzeck has tonal music in the tradition of Mahler, but also some atonal music (music which is not based on any key) as well as melodies which are based on the whole tone scale.  The music sounded very modern at the time it was written.  Berg also writes for the voices in unusual ways: sometimes they have to half-speak, half-sing (this is called Sprechgesang).