The laser kit that I have purchased is the JTechPhotonics - 2.8W Laser engraving & cutting Kit. This comes with both a laser and the power supply with some control electronics.
My goal in driving this laser is to be able to use as much of my current workflow as possible (e.g. Sketchup and Simplify3D), without making any firmware changes on the printer, and with a minimum of hardware intrusion.
The first thing that this means is that I need a way of telling when the laser should be firing and then I need to be able to fire it from GCode. Telling when the laser should be on and off is relatively easy assuming that I write a preprocessor that can scan and regurgitate GCode.
Driving the laser from GCode is a little harder. This is where the power supply and control electronics come in from JTechPhotonics. It can take as input either the LCD or the FAN PWM signal from the printer, both of which are controllable via GCode, and use that signal for turning the laser on and off.