GDevelop's an open-source HTML5 game creation tool.

Homepage: http://www.compilgames.net
Why should we include this: It's an additional tool for game development, and seems to be different from most tools (seems to be programming language-less). Can't hurt to have more game-dev stuff in the repos!
Is it open source?: Yes. https://github.com/4ian/GD/archive/4.0.95.tar.gz