This line following robot is really small and can run on top of desk. Robot senses a line with six optical sensors mounted at front of robot. It also adjusts speed (using PWM) of motors depending on line conditions on order not to loose line due to friction.
This cool line following robot is controlled by ATmega8 microcontroller which is powered from lithium coin cell. Robot uses two separated coin cells to power AVR and motors separately in order to avoid accidental resets. Robot dimensions are 45x33x12.5mm and it weights about 15 grams. It runs at 53cm/s average speed. Firmware and circuit is available for download. Really cool small robot to have buzzing around on your table.