Mirrors lined with blue and red LEDs have long been a staple of Portal-themed decoration. Taking it to the next level, however, is jamin101wolf. He bought the Portal gun prop available through ThinkGeek and Amazon, then plugged the two mirrors into remote-control power boxes. With this he pulls off the effect of firing a red and blue portal (which are located on opposite walls, creating the requisite hall-of-mirrors effect.)
The remote control is not built into the Portal gun; as we speak, a more skilled modder may be integrating it with the toy's trigger. jamin101wolf said he didn't want to risk destroying his. Still, it's a neat trick and one I wish I'd thought of first.
Portal Mirrors Build [imgur. h/t Victor B.]