.NetMap

Microsoft released already a while ago the GLEE library as a closed source library for graph layout which you can use for non-commercial purposes for free. Now, more recently, the MS research group also released on CodePlex .NetMap which

…is a pair of applications for viewing network graphs, along with a set of .NET Framework 2.0 class libraries that can be used to add network graphs to custom applications…

The code is partially based on the GLEE library and is well-documented to allow one to extend it with, for example, additional layout algorithms. So, this library could be a good start for a stand-alone component or library for diagramming in general. The only minor point at this moment is the fact that it’s .Net 2.0 code and, hence, GDI+ based. No WPF in sight. Not yet, but who knows…

See also this post on VS2010 boasting some more diagramming representations.