UMLet is a free, open-source UML tool with a simple user interface: draw UML diagrams fast, produce sequence and activity diagrams from plain text, export diagrams to eps, pdf, jpg, svg, and clipboard, share diagrams using Eclipse, and create new, custom UML elements. UMLet runs stand-alone or as Eclipse plug-in on Windows, OS X and Linux. (Also, check out its sister tool PLOTlet to create chart grids and our other tools.)

via UML Tool for Fast UML Diagrams.