ArgoUML
ArgoUML is a modelling tool that helps you do your design using UML.

Jug
Jug is a tool for converting Java .class files into UML class diagrams. It can read .class and .jar files, and output .emf or .eps files of the generated diagrams.

UMLet
UMLet is an open-source lightweight Java tool for rapidly drawing UML diagrams with a sound and pop-up-free user interface. UMLet lets you draw diagram sketches fast; export diagrams to eps, pdf, jpg, svg, and system clipboard; share diagrams using Eclipse 3; and create your own custom graphical elements.

UMLGraph
UMLGraph allows the declarative specification and drawing of UML class and sequence diagrams. The current features are part of an ongoing effort aiming to provide support for all types UML diagrams.

Violet
Violet is a UML editor with these benefits: It is very easy to learn and use; It draws nice-looking class, sequence, state, object and use-case diagrams; It is completely free; It is cross-platform.