I can't remember where I heard about it, but I stumbled across a prototyping tool last week called Balsamiq Mockups.
It's a simple tool for creating screen mockups, and I had some UI work to sketch out so I thought I'd download it and give it a try. Thirty minutes later I had bought the product ($79 US) and continued to use it to flesh out a UI layout.
I love this tool! It's easier to deal with than using just HTML or a graphics editor like Photoshop. It's much, much more lightweight in both cost and megabytes than something like Visio.
What I really like, though, is the look of the diagrams - they look like sketches on paper or a whiteboard. If you're using the tool to show some UI concepts to end users, they don't look at the result as if it's the actual system, worried about the choice of colours or whether a drop-down is used for a particular field. You can create some more elaborate mockups, but I haven't gone down that road yet.
So, all in all, it seems to be a great little tool and I had no qualms about paying for it.