Over the course of this project two major methods of driving a motor were used. The first was way more hacky and fun then the second. 

For the first two generations of the robot, 74HC244 Octal buffer/line driver were used to drive the motors. They are cheap, and by using all 8 outputs/inputs tied together software PWM allowed basic speed control. 

And it worked really well! Except everyone who tested it really wanted to go backwards for some reason. Since every motor driver on digikey/mouser was still 5x more expensive that a 74HC244, we just doubled the number and uses 2 per side! Sadly, by using two the forward and reverse speed became limited by the current sinking capabilities of the 74HC244 and the kindest description of the prototype was sluggish. You can see the dual line drive prototype below.

After going back to the drawing board, we found the L9110s motors drivers on ebay. The L9110s has two inputs and drives one DC motor at voltages up to 9V. They don't require PWM inputs, and logic high and logic lows are all that is required to control it. 

In order to fix the sluggishness a cheap boost convert was added to increase the voltage from the battery to something higher to let the robot move faster. 

An important note: back EMF

Since the L9110s do not have flyback diodes, there is a huge back EMF when the motors change directions. This normally would do bad things to the boost converter. However to both add protection to the boost convert and add an easy way to set the boost converter to the right voltage, a 5V zener diode and LED were added in parallel to the boost power supply. That way when the LED turns on the boost converter is set to around 6V. In addition, they give the back EMF a path to ground not through the power supply. (And it is noticeable, the LED gets a lot brighter when the motors change direction).

Theses Generation 4 rovers were used as part of a Sumo Bot competition and performed fairly well (there was issues with the battery moving inside the holder and causing it to disconnect)

And this is the solution we used for the Generation 5 and 6 Rovers! Each L9110s is less than 50 cents and runs like a champ!