Leadwerks is a semi open-source C/C++, C# and BlitzMax, as well as community-supported extensions for Delphi, Engine. The engine is offered as both a dynamic linked library (DLL) and BlitzMax module.
Absolutely the best game engine for the small indie developer. I have licenses for Leadwerks, Unity Pro and T3D. Guess what. I uninstalled both Unity and T3D lately because in the end I found that Leadwerks gives me stunning graphics in combination with a very easy and straight forward C API.
I highly recommend you to have a look at thins engine if you are willing to program in LUA or C++.