ArgoUML is an open source UML modeling tool that includes support for all standard UML 1.4 diagrams. It runs on any Java platform and is available in ten languages. Its design evaluation and checklists provides valuable help to make sure that your models are well formed. Beside the modeling aspects, ArgoUML has also some nice features like code generation and reverse engineering.
Web Site: http://argouml.tigris.org/
System Requirements: ArgoUML is a Java based application that needs Java 2 JRE or JDK version 1.4 or higher and 10MB of disk space
License & Pricing: Open Source, Eclipse Public License (EPL) 1.0.
Support: User forum: http://www.argouml-users.net/forum/