Ian Miller, a high school senior in Manhatten, Kansas, built his own version of the Pi-Powered Laser Engraver: engravR. Besides the great YouTube video, he also wrote an excellent tutorial on Makezine.
In the tutorial, Ian included a great circuit schematics of the entire system, which I didn't have in my post. So everyone trying to make a similar project is strongly recommended to visit his tutorial and his blog pxlweavr. He used different H-bridge drivers I believe.
My writeup is here http://funofdiy.blogspot.com/2013/10/a-raspberry-pi-controlled-mini-laser.html