2D Game Engine
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
3D .NET engine for Windows, Linux, Mac
Essence : Ultima Developper Kit
c# lua bind with unity3d project for game development
XNA based game engine completely written in c#
A cross-platform game framework based on OpenGL and GObject.
GameWizzard is a Sprite editor.
Fully fledged steam community SDK
Investigo - DX9 Performance and Debugging Toolkit
Multi-Language Wrapper Collection for the Irrlicht Engine
A framework for games with defense towers along a path.
DREAM is an Open Source GCS