Here is an epic LEGO Portal gun that is fully functional. The gun took a year to plan, and 3 weeks to build. It has just under 2,000 pieces, and has a LEGO Mindstorm controller to help open and close the prongs and turn on the light effects.
Check out the video of the gun in action:
I love seeing it in action, but I personally don't have the patience to build something like this. What would you build if you had the time and inclination?