In the SNES version of FFIV, I used to get a kick out of naming Cecil "Boob", so that when you go to Mt. Ordeals and fight the Cecil doppleganger, Tellah yells out "THERE'S TWO BOOBS?". I thought it was very clever at the time.
Also, my girlfriend once traded me a Drowzee that she had named DrewCarrey. She swears up and down that the resemblance is there, but I really don't see it. In exchange, I traded her a Magikarp named Tittyfish. We had a good laugh.
Finally, I created a new town in Animal Crossing specifically for abusing the system (using the internal clock to time travel, misusing gift codes, etc) which I named Cooterville.
I'm very mature.