John Romero in DOOM II (warning spoilers)
Famously Jhon Romero is best known for putting himself in as the ultimate boss in Doom II, know in the game you actually need to use a noclip cheat in order to clearly see him, else it is just a giant picture of a demon on the wall that shoots other demons out of its head.
Here is a video from youtube that shows John Romero up close as the utimate boss from HELL!