I finally got around to writing up my first Arduino project.
Its an ultrasonic rangefinder that is positioned above my desk at work. I've got some buttons to adjust the detection range. When it detects a state change, ie "Here" or "Away", it sends a signal to a simple Processing app, which in turn hits a web service and logs me as at my desk, or away from it. Then people can visit a web page to see if I'm there or not!
Not terribly useful, but it was fun. :)