I just wandered across the easter-eggish “Reign of Grelok” text-adventure hidden in Fallout 3. I am amused, verily.
Do games like Reign of Grelok rate a spot in the IFDB, I wonder? I wouldn’t mind writing a review 
I think they do.
Where is it hidden?
The Hubris Comics building, in the (I think) lowest level. In through the front door, around over the barricade to the left, through some corridors and down through a collapsed floor.
Really, I guess it’s only “hidden” in the sense of “geez, there’s a lot of wasteland to pick through and … oh look, one terminal has a text-adventure game on it!” 
Once you find the building it’s in, they even point you towards it (the terminal in the reception area teases that there’s a computer-game being tested elsewhere in the building).