Screenshots
Gaphor is a very usable application. It works on Unix (Linux/FreeBSD), Windows and Mac OS X.
The screenshots are a bit outdated, the options in the lower-left toolbox have been extended with options for Class, Component, Action, Use case and Stereotype diagrams.
Gaphor has loaded it's UML meta-model. Gaphor can alter its own data model.
Gaphor features a built in python console, this way one can query and modify their model (and the rest of the application if you like).
Gaphor works nicely on OS X, using the X11 environment. Now we have to wait for a native OS X port of GTK+.
Gaphor is running on Windows XP here. It was not easy, but it can work.