Manuskript - Open-source tool for writers.
Twine - Twine is an open-source tool for telling interactive, nonlinear stories.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
RenPy - Ren'Py is a free and cross-platform visual novel engine that helps you use words, pictures, and...
Storyline Creator - An elegant tool for authors to write, organize and visualize their stories.
Quest - Quest lets you create sophisticated text-based games, without having to program.