With the advent of the diode laser, coherence is finally available at a hobbyist price. Search eBay for “laser module” and you will find lasers from $5.00 and up. There are basically two kinds of laser displays: screen effects, where the laser shines on a screen (which is what we are doing here), and beam effects, where the laser’s path though the air is visible. Beam effects require much more powerful lasers than we are dealing with here, fog machines, and have all sorts of legal regulatory ramifications (see below).
There are 2 different kinds of lasers to look for, diode lasers, and DPSS (diode pumped solid state). The red and violet lasers are diode lasers; the laser light is produced directly by a diode, much like an LED. Green lasers are Diode-Pumped Solid State (DPSS): an IR laser diode pumps a YAG crystal that lases at 1064 nanometer. This passes through another crystal that doubles the frequency emitting the 532nm (green) output. DPSS lasers are available in many colors, but only the green ones are affordable to folks like us. Green will get you the best output for your buck. Not because they are inherently more powerful than reds, but because the human eye is more sensitive to that color range. So a 5mw greenie will be perceived about as bright at a 10mw red laser. The violet lasers are problematic. They are often referred to as “blue” lasers, because they were removed from Blu-Ray players. Their color is almost into the ultra-violet range, and is hard to see at low power. They work best projected onto a fluorescent material such as cloth screens and to a certain extent, photo background paper.