Here is another interesting Arduino-based project that we're surprised hasn't been already published sooner - converting the gear change system on a bike from the cable-pull system to completely electronic. However Nabil rose to the challenge and made it happen with an Arduino, some electronics and a strong servo. The control buttons for the gears are mounted on the side of the handlebar as such:
with the servo neatly mounted on the derailleur:
The circuitry is contained in a box over the centre of the handlebars. This project offers a lot of scope for improvement over time, including storing data in the EEPROM for gear position data or even data logging over time.