Undum is described as 'game framework for building a sophisticated form of hypertext interactive fiction' and is a Game Development Tool in the Development category. There are more than 10 alternatives to Undum for a variety of platforms, including Windows, Linux, Online / Web-based, Self-Hosted solutions and Mac. The best alternative is Twine, which is both free and Open Source. Other great apps like Undum are Ink by Inkle (Free, Open Source), Chronicler (Free, Open Source), Inform (Free, Open Source) and ChoiceScript (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).
Inform is a free app for creating works of interactive fiction, available for MacOS, Windows, Linux and Android. Inside it is a powerful programming language based on English language text. This redesigned website is part of the preparations for open-sourcing the software.
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.