articy:draft 2 is a collaborative environment for the creation and organization of game content like quests, interactive dialogues, characters, items and level layouts – from first design to export right into your game.
Chat Mapper is a branching dialog editor from Urban Brain Studios with Lua scripting and XML export. Non-Linear Branching Tree Graph Visualization, Conversation Simulator using TTS voices, and multiple export formats for printing actor scripts and for getting dialog into various 3rd-party tools like Dialog System for Unity.
It is free to try, with pricing that ranges from $65 for an Indie License, $495 for Commercial, and an If You Have To Ask You Can’t Afford “Publisher” level which includes a player for Unity.
MakeHuman is an open-source human figure creator based on Python. It is free and runs on Windows, Linux, and osX.
Twine is an open-source tool for telling interactive, nonlinear stories.
Twine publishes directly to HTML, so you can post your work nearly anywhere. Anything you create with it is completely free to use any way you like, including for commercial purposes.