Algorithmic composition with MIDI export
If you are willing to spend the time and have a background in programming, this is a great opensource cross platform audio language.
SuperCollider is an environment and programming language for real time audio synthesis and algorithmic composition. It provides an interpreted object-oriented language which functions as a network client to a state of the art, realtime sound synthesis server.
Press Here to listen to or download SC140, an album of 22 pieces programmed entirely in Supercollider by artists from around the world, each piece created with just 140 characters of code.
Moonbell is loaded with several features, good for experimental work.
One of many Mac software applications on this great music site.