One of the most impressive scores ever produced, Cornelius Cardew's Treatise is 193 pages of graphical notation hinting at music to be performed. The instructions for the ensemble are "for any number of musicians with any instruments, may be performed in whole or in part." Sonic Youth and the Ensemble 303 have recently popularized this piece to some extent. An early, 1967, recording by the QUaX Ensemble (highly regarded by Cardew) is now available again as well, see amazon link below.
I won't repeat information easily available online from A Young Person's Guide to Treatise, probably the best single spot for information both on Cardew and Treatise itself. You should check your local library for a copy of Treatise and Treatise (Handbook).