We all know that the TF2 one.. Doesn't fit with the very few portal servers, and stuff like that. We all know the Portal turret is epic. So I'd say somebody should make one.

It shouldn't be too hard..
-Turnable, but unwalkable
-Sounds from Portal 1.
-Animation opening and closing where the bullets come from (the sides).
-Small but not effectable light for the eye