UMLet is an open-source UML tool with a simple user interface: draw UML diagrams fast, export diagrams to eps, pdf, jpg, svg, and clipboard, share diagrams using Eclipse, and create new, custom UML elements.

Much like Firefox, GIMP’s strength lies in its plugins, which are developed by the open-source toting community. Since the users themselves develop them, they know all the needs and conceive a plugin for everything.