Option 1:

Sabertooth 2 x 25

http://www.dimensionengineering.com/datasheets/SabertoothDIPWizard/start.htm

Option 2:

https://www.pololu.com/product/2393

Option 3:

http://www.robotmarketplace.com/products/roboteq_speed_controllers.html

Other related:

http://www.open-electronics.org/here-comes-the-open-source-snow-plow-robot/

http://www.open-electronics.org/ (cadvision)