On the Freebee list:
Penultimate, Pencil, Mockflow, Framebox, Hot glue
And don’t forget… pen and paper
Yes. An actual pen. And some real made-from-wood paper. Okay, so these don’t allow you to make a prototype, and there’s no built-in elements. But, if you feel more comfortable using a more traditional approach, why not get your ideas down on paper first and refine them in software later?