Aidan Lane from the Centre for Electronic Media Art, Monash University, has released Nodal, a generative software application for composing music.
From the Nodal page:
It uses a novel method for the notation and playing of MIDI based music. This method is based around the concept of a user-defined graph. The graph consists of nodes (musical events) and edges (connections between events). You interactively define the graph, which is then traversed by any number of players who play the musical events as they encounter them on the graph. The time taken to travel from one node to another is based on the length of the edges that connect the nodes.
Nodal is based on an original concept by Jon McCormack. It is free for personal use and available for Mac OS X based computers (Universal Binary).