A number of composers around the 'blogosphere are responding to Lawrence Dillon's question: What pieces from the 70s, 80s, 90s and 00s have changed the way composers think about composing?
I don't know what pieces may or may not have changed the way composers composed, but I do have some idea for myself of the pieces that changed how I heard/studied/experienced/composed/performed music. The pieces came to mind based on when I heard/performed/studied them, rather than when they were written. Here then are lists from the '60s and '70s (as Rodney Lister suggests, it is somewhat harder to think of these kinds of pieces from the '80s and '90s).
Ligeti: Atmospheres, Lux Aeterna
Stockhausen: Hymnen, Stimmung
Feldman: False Relationships and the Extended Ending, The viola in my life, Rothko Chapel
Carter: Concerto for Orchestra, String Quartet 3
Lutoslawski: Livre pour orchestre, String Quartet
Satie: Pages mystique
Ives: Symphony 4
Beethoven: Quartet in B-flat, Op. 130
Webern: Sechs Stucke, Op.6
Riley: In C
Reich: Clapping Music, Drumming, Music for Mallet Instruments, Voices, and Organ
Berio: Sinfonia, Sequenza V