If you're into hobbyist robotics or just want to keep track of your Roomba progress, this is an interesting project. John Graham-Cumming converted an inexpensive optical mouse into a robot odometer.
John's robot uses an Arduino Uno where wires from the optical mouse circuit board are attached. It now operates as a stand-alone optical sensor unit. Full details can be found at the source link below.
[John Graham-Cumming via Make Zine]