I thought you all might enjoy a little toy I built called a Reaction Time Tester. I built it as an Arduino learning project, but it's turns out to be pretty fun to play with!
You press a big red button to start play. When the green light comes on, you release the button as quickly as possible, and the RTT displays the number of milliseconds that it took you to react. Repeat 10 times, and your "score" for the "game" is the cumulative sum of the individual response times.
Your goal, of course, is to try to minimize your score. At the end of the game, the top all-time scores (stored in EEPROM) are displayed on the display panel. If you are lucky enough to get the low score, LEDs flash admiringly.
For plans and more details, surf over to the playground:http://www.arduino.cc/playground/ComponentLib/ReactionTimeTester