Glitch Sequencer is a free cellular automaton based MIDI sequencer for the PC and Mac. It offers an interesting style of composition based on evolving patterns and is great for fast, fluid experimentation. Glitch Sequencer was written by Bret Truchan in Processing using themidibus MIDI library.

Glitch Sequencer is based on Conway's Game of Life, which is covered in detail at wikipedia. In practice, you don't need to know much about the details. Start by drawing a "seed" pattern on the seed sequencer grid. For each step of the sequencer, the pattern evolves following a simple set of rules. Once the pattern has reached a certain length, the evolution resets to the beginning state.

MIDI events are generated when specific cells of the pattern change from black to white. You tell Glitch Sequencer which cells should generate which events on the Triggers screen. Drawing on the trigger grid determines when an event should be sent. The Value Sequencers (which look like a bar graphs on the Triggers screen) determine what note should be sent.