MonoUML is a CASE tool based on the mono Framework. Designed for allowing to all UNIX/Linux developers faster computer systems design using a friendly GUI application. Not only a diagramming tool but rather a complete CASE tool based on the OMG standards
nUML is a library for manipulating UML 2.0 models. It works with Microsoft® .NET Framework, Mono, and DotGNU.
Serialization to and from XMI 2.1 is supported.