1. The knocking machine: "You put it on a table and knock on the table: the device detects the knock through a shock sensor in the bottom, copies and repeats it till you hold the hammer to stop it."
2. Beautiful barcodes - that actually work.
3. FlickrGraph - is there no end to the wonderful windows that Flickr users are opening onto that service?
4. Planes that never got built.
5. Networked pac-man - a lovely mod to the original game. Coincidentally, we've just implemented a similar sort of thing for a client using Bluetooth; maybe I'll be able to post about it one day...