The free UML tool UMLet lets you draw UML diagrams with a light-weight, pop-up-free user interface. It lets you draw diagram sketches fast; export diagrams to pdf, eps, jpg, gif, svg, bmp, png, and system clipboard; share diagrams using Eclipse 3+; and create your own custom graphical elements.
New in version 12.0: Automatically create UML diagrams from Java source code or class files; new graphical element types (beta), with syntax completion.