Open Source UML Tools

Does anyone know of any good open source UML tools? I've only used 4 UML tools: IBM Rational Rose, IBM Rational XDE, Microsoft Visio, and Sparx Systems Enterprise Architect. I despise Visio, XDE is sufficient, Rose is pretty good, and EA seems to be the best (although I haven't done a lot of work with it).

Anyway, I have a few ideas about coordinating modeling and coding, but wanted to test it out before I blurt it out to the world - I don't even know how feasible it is, honestly. In order to do that, I would like to reuse an existing UML tool to extend it to add the features I have in mind. I looked for open source tools, and only found ArgoUML. Of course, ArgoUML is in Java and I would prefer a C#/.NET app. I can do it in Java, but would prefer not to.

If anyone knows of a good open source tool...or, at least one with basic class diagramming capabilities, I would appreciate the help! The more advanced the tool, the better (to an extent), but I just want to test out some basic theories. Thanks in advance.

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