Chronicler is an open source desktop application which allows users to create CYOA/MCG (choose your own adventure/multiple choice games) interactive fiction using a GUI not dissimilar to Twine (i.e. in the style of a flow chart).
ChoiceScript is a simple programming language for writing multiple-choice games (MCGs) like Choice of the Dragon. Writing games with ChoiceScript is easy and fun, even for authors with no programming experience.
Inform is a design system for interactive fiction based on natural language. It is a radical reinvention of the way interactive fiction is designed, guided by contemporary work in semantics and by the practical experience of some of the world's best-known writers of IF.