Nodal is a free OS X application which allows one to design their own note generating system – outputting data to any MIDI synthesizer software such as Garageband. –

Nodal is a generative software application for composing music. 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 generative music software [via Create Digital Music]

Online generative music … generator
Computational, generative art