There are many uses for even a 1 mW helium-neon laser. Most of these same sorts of things can also be done with a collimated diode laser (though some laser diodes may not have the needed coherence properties for applications like interferometry and hologram generation).
Below are just a few possibilities.
(Portions from: Chris Chagaris (pyro@grolen.com).)
- Basic optical experimentation such demonstrating the principals of refraction, reflection, diffraction and polarization.
 - Interferometer construction: With a small laser and a few simple optics, this device will allow you to perform many interesting experiments.
 - Free-space optical communications: using some basic electronics for modulation.
 - Fiber Optic Experimentation.
 - Viewing of holograms.
 - Hologram generation: However, I’d suggest a slightly higher powered laser for this.
 - Construction of a basic laser light show. A higher power laser could be substituted when budget constraints allow. 🙂
 - Laser surveillance.
 - Laser tachometer.
 - Laser burglar alarm.
 - Laser gyroscope.
 - Studying fluid dynamics.
 - Applications in construction for calculating distance, leveling, aid in pipe laying, etc.
 
