Monster moto shield Arduino 2 x 30A VNH2SP30 full-bridge driver
2 x 30A VNH2SP30 full-bridge motor drivers.
Has thermal shutdown and under voltage and over voltage protection.
This unit is really a very capable highly upgraded ardumoto - full bridge instead of half-bridge driver that potentially can run upto 30A (14A continuous) per channel
Please see the link page below for more information.
The unit will run continuously and be barely warm at lower currents 6A-8A The driver devices used are very capable so without the screw terminal blocks soldered in and the motor wires direct to the board it is possible to drive higher currents more reliably.
Feel free to refer to the sparkfun page where there is loads of other information
https://www.sparkfun.com/products/10182