Denim is a really wicked cool project that the Group for User Interface Research at UC Berkley have been working on. I’ll be a little lazy and quote Branden from his posting (Its late here, ok? ;-):
Denim is a pen-based Java application that you use to design working prototypes of web sites that consist entirely of sketches. You just sketch out the pages, draw lines between them, and they work!
It is based on a SATIN, a toolkit that they developed for making pen-based applications in Java. Very cool stuff, be sure to check it out.
[Via Branden Hall]