Pulse Width Modulation or PWM for short, is the way to control a DC Motors speed. Using an H-Bridge will allow you to flip the polarity to the motor so you can change direction (Forward/Reverse).
Check out Intro To PWM for more info.
Ok, now you know everything about PWM. So we have to figure out what devices we need to drive the motors. I have researched various options and decided on the popular National LMD18200 3A, 55V H-Bridge chip (Data Sheet). Since we have two motors (one on each side), we need two chips or channels.
The board will have to be controlled by an external microcontroller. This design allows the user to select an appropriate microcontroller that they are familiar with. It is always a good idea to design modular equipment that can be used in a variety of ways. Flexability is a key point in design work.
I have designed a dual LMD18200 circuit board that will allow you to easily control your robot's motion from any microcontroller that is capable of PWM (most like the PIC, ATMEL, OOPIC, etc are capable). If you would like to build your own board, I have a complete set of plans including parts list, circuit schematic, build notes, and pcb layout. I am selling the package for $5.99 US plus $2.25 shipping and handling. If you are interested click here to contact me. I accept cheque, International Money Order or PayPal