Starting today you can draw UML diagrams in Astah (Community) and display the diagrams in Confluence. This way you have a good alternative for the built-in Gliffy editor that uses Flash and works in your browser. Create your UML diagram in Astah, create/modify a page in Confluence and start editing the page. First attach your Astah diagram to the page:
Using the Macro Browser (Insert->Other Macro's) you can insert your diagram and your diagram is on the wiki: