Youtube user kss5095 built a real working model of the turrets from Portal in his Advanced Mechatronics class at Penn State University.
The turret actually tracks a person moving across a room and shoots foam bullets. Sorry, no real guns are mounted on the device.
Although the device comes equipped with a laser, it actually uses a webcam to see its prey. The software controlling the device is written in MATLAB. He got the USB Turrets from Dream Cheeky.
But the thing that makes it really special is that it plays the creepy sound effects from the game when booting up and searching for a victim.
"Would you come over here?"
"Are you still there?"[h/t: ZDNet]