Stride (formerly Xenko), a free and open-source cross-platform C# game
One framework for creating powerful cross-platform games
Vivid3D is an open-source 3D engine with powerful 2D UI.
Modern 3D engine and IDE written using C# and C++.
a 2D Game Development Framework
XNA based game engine completely written in c#
Open Source Virtual World and MMOG Development Platform
A 2D Side Scrolling Platformer Game