Gaphor is a very usable application. It works on Unix (Linux/FreeBSD), Windows and Mac OS X.
Some screenshots are a bit outdated, the options in the lower-left toolbox have been extended with options for Class, Component, Action, Use case and Stereotype diagrams.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 0.17 has a nice multi window interface.
Gaphor 0.15 has loaded it's UML meta-model. Notice the customized drawing style.
Gaphor 0.3 works nicely on OS X, using the X11 environment. Now we have to wait for a native OS X port of GTK+.
Gaphor 0.16 sports extensive support for stereotypes. Stereotypes can contain attributes which can be defined in the typed classes.
Gaphor 0.3 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 0.5 is running on Windows XP here. It was not easy, but it can work.