ArgoUML is a UML diagramming application written in Java and released under the open source BSD License. By virtue of being a Java application, it is available on any platform supported by Java.
In 2003, ArgoUML won the Software Development Magazine's annual Readers' Choice Award in the "Design and Analysis Tools" category.
ArgoUML does not yet completely implement the UML standard and is lacking full support for some diagram types including sequence diagrams.
StarUML - The Open Source UML/MDA Platform running on Win32 platform.