UML Design Tool (Free, with C# Source)

Windows Forms : Control Gallery : UML Design Tool

"UML Design Tool .
It can tell how to write a Rational Rose by yourself
It have powerful edit ability
It's very good example to learn GDI+, UI Design ,Design Pattern and so on"

This is a pretty sophisticated application for creating UML diagrams (Use Case, Static Class, Deployment, State, Component, etc).

The UI appears to be translated into English, and there are some non-English elements, but who cares about that when you have the C# source? :)

Use this for the UML or as an example of using System.Drawing/GDI+ to create user driven diagrams. It looks like there's no external references/libraries so everything you see has source available.

Not only is there diagramming, but the UI had Visual Studio like panels, Office like menus, etc.

Pretty impressive...