A free rendered GUI library providing windowing and widgets for the easy creation of HUDs for games. It supports themes/skinning, XML-based configuration, and works with any graphics API or rendering engine including Axiom, Irrlicht, and SDL.NET.
SAGE stands for Simple Asynchronous Game Engine and is an attempt to implement a multithreaded game engine in C#. For rendering/physics/sound/etc it uses(/will use) open source libraries (like OGRE, ODE, etc) through plugins.