In order to determine the distance a robot travels, John Graham-Cumming found inspiration right on the desktop - and converted an old optical mouse sensor into a simple and usable odometer. After hacking out the sensor part of the PCB, he ran the signals from the mouse through a logic analyser to determine how the mouse reported back to a PC. With this knowledge he then wrote some Arduino code to clock the mouse and retrieve the data - making use very easy.
The only consideration is the surface must be similar to that when using an optical mouse, otherwise the sensor will misread. Nevertheless, to get started visit John's project page here. And 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: