Ficdown is described as 'set of standards using the Markdown formatting syntax that can be used to create choice-based interactive fiction' and is a Game Development Tool in the Development category. There are more than 10 alternatives to Ficdown for a variety of platforms, including Windows, Linux, Mac, Self-Hosted solutions and Online / Web-based. The best alternative is Twine, which is both free and Open Source. Other great apps like Ficdown are Chronicler (Free, Open Source), ChoiceScript (Free, Open Source), Inform (Free) and Ink by Inkle (Free, Open Source).
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.