Rick Bickle of Integrated Control Systems and the Dallas Personal Robotics Group has written a tutorial on using a 555 timer to generate Pulse Width Modulation (PWM) signals for motor speed control. If you're interested in a building a robot that doesn't include a microcontroller, this could be a helpful method of controlling the motor speed. "PWM is the most common method of motor speed control used in robotics. Usually the PWM signal is generated by a microcontroller running a PWM algorithm in software. Rick introduces a way to generate PWM signals with nothing more than the ubiquitous 555 timer and a few discrete components."