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 (distributed under the GNU General Public License)
* It is cross-platform

Violet is intended for students, teachers, and authors who need to produce
simple UML diagrams quickly.  It is not intended as an industrial strength
tool.  Here are some of the features that industrial-strength UML programs have
and that Violet does not have:
* Code generation.  Violet does not generate any source code from UML diagrams.
* Reverse engineering.  Violet does not generate UML diagrams from source code
* Semantic checking of models.  You can use Violet to draw contradictory
  diagrams XMI export or import.  Violet does not generate files that can be
  imported into other UML tools, nor can it read model files from other tools

If you just want to draw simple UML diagrams without too much fuss, chances
are you'll like Violet. If you have more serious needs, check out one of the
other programs.