Reading through Ken Pugh’s Prefactoring I came across a mention of the Napkin Look & Feel Toolkit. It appears to be a very slick representation of a swing environment which looks like it could have been scrawled on a napkin.
Too bad since we pretty much only do web apps I probably won’t get to use it. It could work generically for a UI, but some of the elements like a tree menu are a lot more trouble to put together in HTML. The danger is always that the client, even from a napkin drawing, can get the wrong idea about how the UI will function. Still I might experiment with it despite the danger.