Please see my comment to the question. If you agree to use Java, consider open-source ArgoUML:
http://en.wikipedia.org/wiki/ArgoUML[
^],
http://argouml.tigris.org/[
^].
It is not perfect, somewhat obsolete and minimalistic, but I tried many and for real work use only ArgoUML now. It is convenient and strict enough. I don't use it as an applet, but I see no reason why it cannot be used as applet, as source code is available.
—SA