StarUML is a great open source UML application. Supporting UML 2.0 and MDA (Model Driven Architecture) StarUML let's you work with all related diagrams. Code generation can be done for Java, C++ and C# - and documentation can be generated for all of the Microsoft Office suite programs such as Word, Excel and PowerPoint. StarUML is easily extensible through modules using a COM-based plug-in architecture - and by use of templates allowing you a make new code generators. StarUML even imports from Rational Rose.