Orx is an open source, portable, lightweight, plugin-based, data-driven and extremely easy to use 2D-oriented game engine. It has been created to allow fast creation of games and prototypes. All objects and cameras are created in a 3D space, but only a 2D rendering plugin is currently available (however hooks are provided for those who need to roll out their own 3D rendering).
The data-driven paradigm is vastly underappreciated. It takes some time to understand how to best leverage it, but once you've get the hang of it you have a very powerful toolset. I see this engine as a shining example of data-driven development.
Really good community, and the engine is designed in such a simple and logical way that you don't feel like you're coding into a corner. The use of objects, events, shaders and the config system work in a very seamless and clean way.
This is a pretty clean engine.