I have been developing a yet-to-be-named game engine on and off for about a year. It uses OpenGL and C++, and was started in a graphics programming class at Becker. Right now it supports deferred and forward shading, kind of. I say "kind of" because both modes have been implemented, but some features are not available in the other. Forward rendering has basic supports for shadow mapping (actually very basic, it only supports one directional shadow map), but deferred rendering does not have shadow mapping. Deferred rendering has support for emissive materials, while forard rendering does not.
In the future, I'd like to add many new features and make this engine far more robust. I've been struggling for a while on what rendering pipeline to use, and I think I've settled on developing both a forward and deferred rendering pipeline. From what I understand, forward is still used for some objects in deferred mode, such as transparent materials, so developing a forward pipeline seems needed anyway. I'd also like to implement more effects, like proper anti-aliasing, ambient occlusion, bloom, actual shadow maps for both modes, anything to make things prettier.
Anyway, here's a video showing the deferred mode. It's running about 100 lights at about 100 FPS on a GTX 780, which screams that it needs optimizations, but it's a start.