Amulet is a free Lua-based audio/visual toolkit suitable for small games and experimentation. It runs on Windows, Mac, Linux, HTML5 and iOS.

Amulet comes with a decent amount of documentation including an introductory tutorial and API reference. You can also ask for help in the forum.
Online editor
Make something right in your browser or explore examples using the online editor.
Open source
Amulet is available under the MIT license, which means you can freely modify and use it for whatever you like, including commercial projects. The source code is available on GitHub.