Axiom 3D Engine
A well designed, flexible, cross platform, and extensible 3D graphics engine development effort using C# and the Microsoft .Net framework.

Netron
Generic diagramming, graph-drawing and graph-layout kit for the Microsoft .Net framework.

ODE.NET
ODE (Open Dynamics Engine) is an open source rigid body dynamics simulation engine. ODE.NET is a .NET wrapper to ODE, built so that the wonderful world of .NET can sample accurately simulated rigid body goodness.

SDL.NET
SDL.NET is a set of object-oriented CLS-compliant .NET bindings for the SDL gaming library and provides high-level access to audio, keyboard, mouse, joystick, TTF fonts, various image formats, MPEG-1 and 3D hardware via OpenGL and 2D video framebuffer.

SharpGL
The SharpGL graphics library is a C# wrapper of OpenGL. However, it is also a fully object orientated Scene Graph. Programming 3D games and applications is made a lot more straightforward with SharpGL, it's framework is very extensible and simple to work with, and all of the sourcecode is free to download and use in applications, even commercial ones.

ZedGraph
ZedGraph is a class library, user control, and web control for .net, written in C#, for drawing 2D Line, Bar, and Pie Charts. It features full, detailed customization capabilities, but most options have defaults for ease of use.