Unity3D Client And C# Server Framework
One framework for creating powerful cross-platform games
A game framework written with osu! in mind.
Molecular viewer and prototyping framework
Vivid3D is an open-source 3D engine with powerful 2D UI.
Modern 3D engine and IDE written using C# and C++.
Mad Vandal's FortressCraft Mod Source Code
Reactive extensions for Unity
3D .NET engine for Windows, Linux, Mac
Gives agents personality, moods, emotions and emotional attachments.
XNA based game engine completely written in c#
Open Source Virtual World and MMOG Development Platform
Investigo - DX9 Performance and Debugging Toolkit