As a simple Arduino-based project and also a fun game, Instructables user "EpicWolverine" has created the reflex-testing game "ButtonHero". In theory the game is easy - there are four buttons with matching LEDs, and you need to press the button matching its LED before then next LED lights. However as you could imagine the period of time decreases after every press, thereby requiring ever-improving reaction time. Here's a quick demonstration:
It's a fun game and with a little more work could be housed in a solid enclosure and more usable for other people. To make your own you can find the Arduino sketch, circuit schematic and notes here. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
So if this sort of project intrigues you and you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: