From the Penn State Web Standards list
For my own reference, this site has some good examples of different interfaces sketched on legal paper (and maybe some cocktail napkins). Also has a video showing a usability test with a paper set of paper prototypes.
While I’m at it, I should add this link from A List Apart
Interestingly the author argues for a “medium quality” prototype because wireframes may be so bare that users may not be able to infer functionality. FYI – The problem with “professional” quality is that users get distracted with suggestions on fonts and colors (been there, done that).
I’ve had some success with the “mid level”, but I’ve seen projects where the bare paper does work, so the key seems to be keeping the concept minimal enough for users to focus on function and not fonts.