A few months ago, I spent some time writing a MIDI sequencer for the Arduino. I finished the sequencer, wrote a couple of songs with it, and then got sucked into MSc thesis mode and had to abandon it.
Well, I've finally got round to publishing it as an open source project on Google Code, so hopefully others will be able to have some fun with it. I've also written some documentation which will hopefully make it a little easier to get started creating music with it. I've named it ‘Cahors,’ which is a word I found in The Deeper Meaning of Liff. See the FAQ page on the Google Code side for a full explanation.
You can hear the first full piece I wrote with Cahors on last.fm here, or you can download the MP3 from the Google Code project page. I've not had a chance to go back and write and more music for it recently, but hopefully I'll get back into that soon.