Peter Hilton wrote a terrific article about doing UML design inside of Confluence, instead of a heavy-weight UML authoring tool. As he so aptly says,

Agile software development needs agile documentation, and drawing UML in Confluence’s Graphviz plug-in is way more agile than fiddling with vector drawing tools like Visio all day.

Find the whole article on the Lunatech site.

Using Confluence for UML; 'Agile development needs agile documentation'