NClass

Open Source / Non-Proprietary

an open source UML-based class diagram editor for the C# and Java programming languages.

NClass is a free, UML-based class diagram editor with full C# and Java language support. The user interface is designed to be simple and user-frienly for easy and fast development. Properties, enums, delegates and other language specific elements are fully supported with strict syntactical and semantical verification, and declaration parsers provide a handy way for skipping the old mouse-controlled editing.

Diagram skins, code generation, reverse engineering and other cool features are already implemented, although many others are on the way. A plugin system provides an easy way to extend the main functionality.

http://nclass.sourceforge.net/