Weta, the minds behind most of the costumes, sets, weapons and creatures seen in the Lord of the Rings films, have made Valve a working Portal Turret. It’s got a working motion tracker, a sleep mode, and the same eerie cadence that made them almost as scary/pitiful as GLaDOS. There's a video below.
Valve's unboxing video of the turret, demonstrate's the turret's working motion controls. It targets Valve employees and then flashes lights at them, rather than ripping them to shreds with bullets. I imagine the turret is a little disappointed by this.
Unfortunately it doesn’t look like Weta will be selling the highly custom made replica, although there are some miniatures being made by Gaming Heads, complete with sound module and glowing lights. Only 350 of the replicas are being produced, however, and that initial 350 have sold out, so any new orders go on a wait list.
Here’s the video, so you can fully engorge your jealousy glands, you crazy angry lizard man.
And here's Cave Johnson explaining exactly how Portal Turrets are made. The more you know.
And here's the Team Fortress 2 turret Weta made for Valve back in 2010. Life is so unfair.