A very nice open source drum machine based on the Roland TR-707. It's released under the GPL, and is quite stable in my experience, even though it is still in early development. I despise the tcl/tk UI, but I still think this program kicks ass. I like to have it control my Juno 106 via MIDI.
Here is its homepage: http://www.vislab.usyd.edu.au/staff/chris/tk707/