ArgoUML - UML modeling tool
ArgoUML is the leading open source UML modeling tool and includes support for all standard UML 1.4 diagrams. It supports Class diagram, Statechart diagram, Activity diagram, Use Case diagram, Collaboration diagram, Deployment diagram and Sequence diagram. The diagram could be exported to GIF, PNG, PS, EPS, PGML and SVG. It generates code for Java, C++, C# and PHP. It also supports reverse engineering, generates code from diagram.