MUDs were ridiculously fun. In college I was starting an LPMud of my own and somehow stumbled into using lex/yacc to alter the room and dialogue descriptions depending on point of view. For instance, a character would leave town to visit a cursed place, and then when they came back to town, they'd find it completely abandoned and threatening. But meanwhile, the other players in the town would experience the character speaking in with bizarrely altered words and speech patterns since the character was possessed. (I did this by taking the Swedish Chef filter and altering it.) But then the university admins found it and deleted it. Argh.

I had a friend whose character figured out how to put on a hockey mask, take out a chainsaw, and then hack off your limbs and give your limbs back to you - this was by coding from within the game, sort of.