In case anyone is looking for a more complete solution, search PlantUML. It supports almost all UML diagrams. I use sequence, class and activity diagrams.
PlantUML uses Java. Theming support in PlantUML is horribad.
I've pushed hard for people to use PlantUML where I work, and some awesome people got it plugged into our mediawiki instance. It's been great for complex functional/design specifications. When someone adds a comment saying "you forgot about case X", you can just press edit, and update your diagram.
It's wonderful and I really do recommend people try it out.
It's wonderful and I really do recommend people try it out.