Ren'Py
The Ren'Py Visual Novel Engine is a free software engine which facilitates the creation of visual novels, a form of computer-mediated storytelling. Ren'Py is a portmanteau of ren'ai, a type of game made using Ren'Py; and Python, the programming language that Ren'Py runs on. Ren'Py has proved attractive to English-language hobbyists; over 500 games use the Ren'Py engine, nearly all in English.
Read more about Ren'Py: Features, Notable Ren'Py Games