s&box is a modern game engine, built on Valve's Source 2
Modern 3D engine and IDE written using C# and C++.
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
a 2D Game Development Framework
A starter development kit that let you develop FF6-like games on PC.
3D .NET engine for Windows, Linux, Mac
Create .xnb files for XNA games.
1st 3D game engine built in C# for .NET, Visual3D Engine's predecessor