Here (video) is a really novel approach to paper prototyping by Chris Neale.

Love the combination of low-fi media with hi-fi communication efficacy. Check out Chris' site for an extended discussion of animating paper prototypes. It's particularly impressive that this type of animation apparently only takes 15 minutes to produce. I've used Flash and OmniGraffle Professional to do the same thing and it took a heck of a lot longer than 15 minutes.