UML in Eclipse
This is the first plugin I've tried for UML and I like it. It doesn't have the polish of say, FlashBuilder, but it is solid, flexible and easy as pi to use:
If installing as an Eclipse plugin, simply download the .jar file, drop in your eclipse plugins folder and restart / startup eclipse. Once in Eclipse, in the Navigator view, right click on the folder in which your diagram will be saved and choose: New -> Other -> Other -> UMLet diagram.
Now, can anyone recommend any great UML books?