Some years ago someone figured out how to build logic gates in Quake 3 maps out of doors and shooters (entities that shoot a projectile when triggered). This was actually used in some single player maps and to create special game modes that aren't present in vanilla Q3.

I guess this could, in theory, be used to create a Turing Complete computer. It might hit the entity limit of Quake 3 though.

PDF: http://www.skore.de/images/stories/Q3A/Q3A_Electronics.pdf