LEDs always seem to inspire clever multiplexing solutions. I got this kit recently and am looking forward to assembling it over the long weekend:
http://jimmieprodgers.com/kits/lolshield/
The LoL Shield is a charlieplexed LED matrix for the Arduino. The LEDs are individually addressable, so you can use it to display anything in a 9×14 grid. Scroll text, play games, display images, or anything else you want to do.
I love the simplicity of the design. Just gobs of LEDs, traces on a PCB and a header to connect to the Arduino. The design & source code are freely available:
More details from the Make blog:
Drew
@pdp7