A game framework written with osu! in mind.
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
Android, DOS, FirefoxOS, Linux, MacOS and Windows all in one..
Log and Play Retro Controller Inputs
Sudoku engine for .NET 4.0 and up
XNA based game engine completely written in c#
Open Source Virtual World and MMOG Development Platform
Investigo - DX9 Performance and Debugging Toolkit