Recently I've been working with a 3rd party diagramming library written in windows forms. It's been great and it's pretty fast in building 80% of the application I'm working on.

[Update August 2012: I never got around to doing much with the diagramming library. I opted instead to re-host the visual studio workflow designer for my needs, why I actually considered writing it is beyond me, I’m a campaigner for bolting existing solutions together rather than re-inventing the wheel ]