Grblshield is designed to work with grbl which is a 3 axis Gcode interpreter (CNC controller)
http://dank.bengler.no/-/page/show/5470_grbl
Be advised that we have adapted the code slightly to work with the shield so you'll want the code available through the Synthetos Wiki
http://www.synthetos.com/wiki/index.php?title=Projects:grblShield
We are using the TI drv8811 chips. They are way more powerful than they look. They can do 2.5 amps per winding and are happiest when running at 24 - 30 volts. They are power-pad chips that are connected to the copper on the top and the bottom of the board for heatsinking. Even so, if you want to run them that high you should fan cool them.
We are also working on a complete embedded controller called TinyG with 4 axes to handle Repraps and the like. It runs USB connected or can also be driven from Arduino serial.
http://www.synthetos.com/wiki/index.php?title=Projects:TinyG
Alden