Vivid3D is an open-source 3D engine with powerful 2D UI.
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
XNA based game engine completely written in c#
2D XNA Game Engine
A 2D tile map editor created using .NET framework.
A game maker written in C# and FreeBASIC
WGFX 2D Game Engine
2D top down arcade shooter
A Game Built with Unity Game Engine
A c# 2D Vector Graphics Library - and more.