Here is an interesting relay that is really two independent relays in one package. 25 amp contact rating, dual SPDT contacts and 53 ma for each coil current. so in this one package a arduino using two digital outputs controlling two small NPN transistors to power the coils, one could program control to go forward, reverse, and break. Add a power N-channel mosfet in the negative side of the external DC power supply for the motor and you can add variable speed via an additional arduino pwm output pin. Lot cheaper and smaller then a 25 amp H-drive. And compared to a solid-state H-Drive you only give up a little on the speed with which you can switch from forward to reverse, which for a robot main drive is probably not an issue? I suspect this is used in electric window motor drive, seat control motors, door lock solenoid, etc.http://www.allelectronics.com/make-a-store/item/RLY-547/DUAL-25-AMP-RELAY-12-VDC-COIL/1.html
So high current H-drive on a budget, what's not to love about that. That lets you put more $$$ into sensors and other cool stuff. I may have to pick up a couple of these.