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
A .NET ascii/ansi console engine written in C# for MonoGame and XNA
Teach retro game development with streamlined workflows
a 2D Game Development Framework
Build a track from wooden blocks for marbles.
XNA based game engine completely written in c#
An open source MMORPG engine.