Inspired by some of the people I just met at the Howdoino event, I went home and finished my first ever Arduino project.
It doesn't look like much - actually, it looks pretty awful and the soldering is enough to get me banned from here - but it works.
It's a prototype circuit for a footpedal/stompbox that when pressed, sends a MIDI command to the computer. So it can be used to turn effects etc. on and off in sequencers and things like Guitar Rig.
One day I might even get around to taking this and making a proper one in an actual enclosure. But for now, I'm chuffed to have done something.