The Irrlicht Engine is a high performance real-time 3D engine written and usable in C++, and also available for .NET languages. It is completely cross-platform, using D3D, OpenGL, and its own software renderer.
Satsuma is an easy-to-use and flexible graph library for .NET. It implements all the necessary graph structures and the fastest possible graph algorithms.
A lightweight spreadsheet program.
A program to create bitmaps from simple ASCII diagrams.