MiniUML is a plugin-based diagram editor, developed as part of a course at the Technical University of Denmark.
Currently, it supports only UML Class Diagrams (as seen in Visual Studio), but it could theoretically support just about anything.
The application is based on C#, .NET Framework 3.5 and Windows Presentation Foundation.
These details are provided for information only. No information here is legal advice and should not be used as such.