Big list of rapid prototyping tools

iPhone and iPad prototyping with taps, hold, swipes and transitions

Starter for a responsive layout if you like grids

Efficiency, emotions, collaboration, interaction, testing

Interesting, but not sure it's quick enough to use... Encourages attention to detail in the prototype, at the risk of getting stuck to solutions too early in the process.

Make touchable prototypes from images, for Nokia touch

Online prototyping and site building service

Online wireframing tool

rapid prototyping tool, create documentation on the fly