UML CASE tool - UML diagrams, use case modeling, reverse engineering and more...
BS Editor: Visual Paradigm for UML (VP-UML) is an easy-to-use, full-featured UML modelling tool that supports full software life-cycle - analysis, design, construction, testing and deployment. This UML modeling tool helps you build quality applications FASTER, better and cheaper. You can draw all types of UML Diagrams, generate code from UML diagrams and vice versa, and generate documentation. This UML CASE tool also provides plenty UML resources - UML tutorials, sample projects and demos.

VP-UML Features:
+Full UML notation support (use case diagram, sequence diagram, collaboration diagram, object diagram, class diagram, package diagram, state diagram, activity diagram, component diagram and deployment diagram)
+Textual analysis for identifying candidate classes, use cases, actors...
+CRC Card for finding objects
+Use case modeling (use case scheduling, use case description...)
+Business Workflow diagram
+OOAD support - object-oriented analysis (OOA), object-oriented design (OOD)
+Incremental round-trip engineering
+Code Generation - diagram to code, model to code (UML to code, UML model to Java)
+Reverse engineering - code to diagram, code to model (Java to UML models, Java to UML diagram)
+Instant Reverse for XML, DotNET dll/exe, C++, CORBA IDL
+Synchronization Between diagram and source code automatically
+Report generator for generating documentation to HTML/PDF
+Auto diagram layout - arrange classes and connectors in UML diagrams in different styles
+Import/export XMI
+Import Rational Rose file
+Visio Integration - draw UML diagrams with Visio stencils
+Export diagrams to SVG, PNG, JPG
+Template and Plug-in
+More...

Other UML Modeling Tools / UML Plug-ins:
Windows Platform:
+SDE for Microsoft Visual Studio .NET
Java Platform (Mac OS X/Windows/Linux):
+SDE for Oracle JDeveloper
+SDE for IBM WebSphere (WSAD)
+SDE for Borland JBuilder
+SDE for IntelliJ IDEA
+SDE for Eclipse
+SDE for NetBeans
+SDE for Sun ONE
+More SDE... you can free download Visual Paradigm for UML (CE) for Mac OS 6.1 now.