One framework for creating powerful cross-platform games
Vivid3D is an open-source 3D engine with powerful 2D UI.
2D Game Engine
Modern 3D engine and IDE written using C# and C++.
XNA based game engine completely written in c#
2D XNA Game Engine
A 2D tile map editor created using .NET framework.
2D top down arcade shooter
2D Isometric Engine
Isometric engine for 2D rendering with XNA and SDL