Urho3D is a free lightweight, cross-platform 2D and 3D game engine implemented in C++ and released under the MIT license. Greatly inspired by OGRE and Horde3D.
It's free and open source. As a developer I think it's awesome because of not only 2D and 3D capatibility but with room for growth and future development.
Simple, fast and powerful open source engine. Also it has a editor where you can create your levels. Personally I like that besides C++ and lua it supports AngelScript and you can write your games in C++ like language then just save the file and test the changes(no need to recompile the source).
Urho3D is frendly & free & fast
Nice engine. Clean code and active development.