I advice to use Pacestar UML Diagrammer. It helps you generate UML 2.0 diagrams quickly, easily, flexible AND commonly understood notation.
I used it in many projects and I'm very satisfied. And too it doesn't use much of memory and space just 6 Mo of Hard Disk.
And the most feature that I like it very much is that I can copy diagrams from the editor and paste them in MS Word... so when I need to edit a specific diagram, just I click on and it will be opened in the editor and by closing it, the updates had been done in MS Word document.