Ren'Py

What is Ren'Py?

Ren'Py is a visual novel engine that helps you use words, images, and sounds to tell stories with the computer. These can be both visual novels and life simulation games. The easy to learn script language allows you to efficiently write large visual novels, while its Python scripting is enough for complex simulation games.
Ren'Py is open source and free for commercial use. It supports Windows, Mac OS X, Linux, and Android.
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How do I get Ren'Py?

The latest release of Ren'Py is 6.12.1 "Human Factor", originally released on May 02, 2011. It was updated to version 6.12.1.1520 on June 23, 2011. Releases are announced on Twitter and Launchpad.
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Where do I start?

A comprehensive manual, including a quickstart guide, is included with Ren'Py and available online.
Our wiki includes user-contributed documentation, including a FAQ, code examples, and interface translations.
The best place to ask questions about Ren'Py is the Lemma Soft Forums. We also have an IRC channel, and a bug tracker.
You can also contact us via email.

Credit:http://www.renpy.org/