s&box is a modern game engine, built on Valve's Source 2
Stride (formerly Xenko), a free and open-source cross-platform C# game
Cross-platform 2D game engine focused on ultimate productivity
Open source reimplementation of GTA San Andreas game engine in Unity
2D Game Engine
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
Sudoku engine for .NET 4.0 and up
A 2D tile map editor created using .NET framework.
WGFX 2D Game Engine