Open Game Engine 2D is a free, open source, full
features, and
cross-platform 2D game engine.
It aims to help developers to create 2D games via only flat files
(readable files such as ini and script files),
so that games could be written only once and played under any platform
supported.
The official site of the project is http://oge2d.sourceforge.net.
And there is also a game design tool made for this project, you may
find it on http://ogm2d.sourceforge.net