Rainbow is a scriptable, cross-platform, 2D game engine written in modern C++.
Ranked in these QuestionsQuestion Ranking
Pro Open source
Distributed under the MIT License.
Pro Written in modern C++, is scriptable in Lua
With support for Android, iOS, Linux, Mac OS X and Windows.
Pro Optional FMOD audio engine
Box2D integrated with Lua bindings.
Pro Supports Spine
Con Not as feature-rich as other engines
Con Small community
Being fairly new on the scene, there aren't much of a community to speak of.