This is a very interesting project, the goal of which is to make prototyping physical devices a fairly straightforward process. The technology, in this case, takes advantage of both hardware and software components. Watch the video on the page -- it takes just a few minutes, but explains the project well. And if you are like me, you'll be really impressed with what they are doing.
I bring this up, because software prototyping is still something I think we can do better at. I've seen realatively good results by using photoshop, plain HTML, or even recently MS Frontpage (thanks to Tim). While these approaches kinda work well for representing a simple GUI mockup, it would be nice to be able to wire a bunch of GUI mockups together easily to form a prototype of some kind.
So what do you use to prototype and mock stuff up?