These seem to be growing in popularity to build lately. Here is another write up to build on with an arduino to control it. I like the box the author built for this too. I like the safety measures the author put in (you lift the lid, the laser goes off). One of the things I am leery about with lasers is the dangers to the eyes it can cause. Looks like a simple 250 mw laser will run about $25. I don’t think I am brave enough to try using the ones from the DVD drives.
http://www.instructables.com/id/MrEngraver-the-Desktop-Laser-Engraving-Machine/