Open Source / Non-Proprietary
a Java-based open source free UML modelling tool, closely follows the UML standard. Can reverse engineer Java code (Plugins available for other languages, inc. c#). BSD license.
ArgoUML is a leading open source UML modeling tool and includes support for all standard UML 1.4 diagrams. It runs on any Java platform and is available in ten languages. See the feature list on the web site for more details.
ArgoUML is a Java based Universal Modeling Language tool. It is able to create and save all standard UML diagrams. ArgoUML also has the ability to reverse engineer compiling Java code and generate UML diagrams for it.
ArgoUML is made available under the BSD Open Source License.
ArgoUML is not production ready! This means that the things you can't do will distract you, you will have problems that you won't have in commercial UML tools, and there is no one to blame for this. On the other hand, this also means that you have the opportunity to fix your problems yourself.
The project team offers daily support on many channels: issue tracking, email, forum, wiki and chat. Depending on your need, some of these possibilities are more suitable then others. Check the web site for details.