Create and maintain UML diagrams in your Google Docs. The diagrams are drawn by PlantUML, a text-based UML diagramming tool, and inserted into your Google Docs as images. You don't need to spend time with the layout of your diagram; PlantUML takes care of that thanks to Graphviz algorithms.
The PlantUML source text is encoded within the link of the image in Google Docs, so you can edit it later and re-insert the diagram using PlantUML Gizmo.
There are some sample diagrams in PlantUML Gizmo to help you get started.