ZZT was an ASCII based computer game, created in 1991 by Tim Sweeney, the creator of Unreal.
Although ZZT's graphics were obsolete before it was even created, ZZT managed to become fairly popular because of its integration of a simple but effective object-oriented scripting language known as ZZT-oop. At the time this was groundbreaking as most functionality in prior games had been hard coded. The language allowed extensibility that no other game was able to provide, and allowed a large degree of community involvement that extended far beyond simply creating level terrain with the excellent editor but involved writing programs to make the game run.
Originally ZZT was shareware, with only one of the four level-sets or "worlds" released without payment. However when the game became obsolete it became freeware with all four worlds released for free. The worlds are: Town of ZZT, Caves of ZZT, City of ZZT and Dungeons of ZZT.
Third party worlds for ZZT are diverse, they range from shoot-em-ups to complex role playing games to a lemmings clone, and far more. They also vary from the simple to the complex, from inane to brilliant and from perverted to enlightening. Many other games have been inspired from ZZT such as Megazeux, and ZZT's sequel: Super ZZT. Many third party worlds can be downloaded freely from the thousands available in vast repositories on the Internet or made easily at home.
Although it has been many years since Tim Sweeney first wrote ZZT in Pascal, it still keeps a place in many peoples hearts. ZZT is still loved by many people have been initiated into programming by it as a stepping stone.