Ren'Py
The Ren'Py Visual Novel Engine is a free, open source, cross-platform authoring system and engine for creating visual novels, a type of interactive storytelling that focuses on text-based interaction with illustrated (but not usually animated) characters through menu-based conversation. As a mature environment, it provides automatic support for several useful features including save/load, branching stories, and visual transitions. Authors create stories with a custom scripting language that can also include custom Python code to extend base functionality. As of 2013 Ren'Py is still widely used and in active development.
Several critically acclaimed and award-winning interactive stories have been created with Ren'Py, including Analogue: A Hate Story (a 2012 IndieCade finalist) and Save the Date (2013 IndieCade).