The registered version of Super ZZT, generally considered a poor man's ZZT. You'll need it to play any Super ZZT games you might find, though.
"ZZT, Super ZZT, Revenge of ZZT, & Best of ZZT."
This is the newest version of ZZT, and it is the one you should use to play and edit games. It includes documentation and the 6 original ZZT worlds (Town, Caves, Dungeons, Underground City, Tour and Demo).