My demo for the 2008 linux.conf.au Open Day is whimsically entitled "Pong Hero". It's a two-player game using the Nintendo Wii Remote as an infra-red camera (inspired by Johnny Chunk Lee's Interactive Whiteboard project), and two home-made infrared light pens. I have a separate page describing how to make the pens.
The code can be grabbed straight from the Ozlabs git repository: it's pretty hacky and has at least one known bug in collision detection.
You can see see a video of the start of a game thanks to Matt Rechenburg, and a longer one with some setup thanks to Paul Fenwick.
Pong hero is available under version 2 of the GNU General Public Licence (GPLv2) or (at your option) any later version.
Thanks to Caitlin for hardware design and construction help, Akkana Peck for the simple images, Val Henson and Anthony Towns for wading into the codebase and finding my bug.