The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++. It is completely cross-platform, using D3D, OpenGL and its own software renderers.
BianLian Framework plan to establish a cross-platform, cross-language, JSF .NET like solution. To make all the GUI (both JSF .net and so on) INDEPENDENT to implementation. PS, BianLian is the famous Chinese art with about 1000 years history.
Enterprise- level HTML5 application development platform
With its familiar .Net visual drag n' drop, WYSIWYG designer and HTML5 rendering engine, Visual WebGui is the natural choice for core Visual Studio developers who want to create native quality, cross-platform enterprise level HTML5 apps in VB.NET or C#.
Superseded by OpenTK: https://sourceforge.net/projects/opentk/
The Tao Framework for .NET is a collection of bindings to facilitate cross-platform game-related development utilizing the .NET platform.
Superseded by OpenTK: https://sourceforge.net/projects/opentk/
hostingstar.com.au's open source software.
To be used primarily for dotnetnuke modules, a crossplatform backup system, .net (dotnet) user controls, 3d plugins and many other bits and pieces.