We are using the Arduino for the underwater robotics competition we host (NURC). Here is a speed controller that will work that will control 2 motors. Need to look at the amps it can handle both continous and peak. This one does 12 amps continous and 25 peak per motor. Hope this helps at least as an example:
http://www.trossenrobotics.com/p/sab...er-for-RC.aspx
The CIMs can take a lot however. For example they are on the 40 amp auto reset circuit breaker. You will have to know your power requirements first. Just make sure also that the pwm signal of the controller and arduino are compatible. Here is another link to look through some speed controllers:
http://www.robotmarketplace.com/products/speed_controllers_main.html