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Gaphor is the best free UML tool for X I've seen yet. It's far from finished but at least it produces pretty graphs. I started writing a C# export plugin for it. The first somewhat working version is available. This has been developed with 0.10.4, no guarantees of it working with any other version. No guarantees of it working with that version either, to be exact, but at least it works for me.

Extract it inside Gaphor's source directory, in gaphor/data/plugins.