Sample Windows Forms application for drawing graphic objects in a window client area using drawing tools and mouse. http://www.codeproject.com/csharp/drawtools.asp
This article explains the myUML project that provides a set of tools for the creation and manipulation of UML diagrams. http://www.codeproject.com/csharp/myuml_1_pack.asp
Netron Graph Library
http://www.codeproject.com/csharp/drawtools.asp