Every time I start a new architecture role, one of my first tasks is to hunt out a UML tool. I'm still constantly surprised at how little traction UML has within the software development industry, with few people using it and fewer people having access to a UML tool. As I type this I'm listening to Grady Booch's 4 part "podcast" series about software architecture and he just said that "a model is a simplification of reality". For me, this is exactly the reason I use UML - it allows me to present the architecture to other stakeholders in a simplified way.
I've been hearing a lot of good things about StarUML, but what's your UML tool of choice?