A Penn State University student has designed a slick Portal turret for his mechatronics class. Now that is one cool course I would have actually loved to take in college.
The turret fires foam NERF-style missiles from a pair of wired launching platforms powered by a USB port. The student also used an Arduino controller and breadboard, along with some custom wiring and Matlab running on a laptop to control the show. Tinted acrylic was apparently cut out to serve as a base for the turret.
The designer notes that the shell for the project remained incomplete but is in the works. Between the two turrets sits a web-based IP camera. One of those turrets also has a small laser pointer attached to the side. It doesn't appear that the laser pointer does anything other than let you know when you're targeted, as the missiles aren't hitting near the red dot as far as I can tell.
As the the target moves, the turret seems to track its movements quite accurately. Once the target stops and is within range, the missile launchers begin to fire their missiles. The turret is also programmed with sound effects, saying things like "come over here" and "where are you."