Need a motor driver that can run at the lower voltages? The Didel 2 Motor Minishield can come to your rescue! Use your Arduino Uno (or form factor compatible) board with it to drive a tiny robot around.
You got some really big motors you want to control? Take a look at the Sabertooth dual 25A motor driver, able to supply 25A each to two motors, at up to 30V. It's a brute, but it's smart enough to talk in many ways!