In C by Terry Riley
In C is an semi-aleatoric (chance music) composition for an unspecified number of players composed in 1964 by Terry Riley. Performance materials include a page of musical fragments along with an explanation of how the piece should be realised.
Many aspects of the performance can be left up to the individual players and the duration of the piece can be varied as well as the instrumentation.
In Ireland, the piece was performed by Greystones Youth Orchestra. at the 9th IAYO Festival of Youth Orchestras in 2004. The score and directions are available at www.otherminds.org. You can read more about the piece at Wikipedia.