EasyDiagram.NET is a self-enclosed ASP.NET 2.0 Diagramming component that is built entirely in C#. It uses AJAX methods for node drawing and node repositioning, and boasts an incredibly responsive redrawing engine without the need for any plugins (such as Flash, etc). A variety of events (such as OnNodeRepositioned, OnNodeDoubleClicked) can be hooked into to ensure seamless server-side integration.
UML (class diagramming), Geneology, flowcharting -- virtually any diagram is possible because the node image and text can be set dynamically.
Both orthogonal and direct-line diagramming is possible, and the component is a draggable from your toolbox onto the webform.