POP (prototyping on paper) by woomoo is a new, free, app to quickly create rough iPhone prototypes. You sketch out a couple of screens on paper, take pictures of them with POP, and link them together by defining tap areas for each screen.

You have a limited amount of projects (two to start with, but you get an additional seven after confirming your mail) and by the looks of it you will be able to unlock more projects through in-app purchase soon. Projects can be shared from within the app as well as from the web—you’ll get a unique URL for each project that can be used to preview it.

The idea behind POP is great, but the implementation is lacking. The problem is that’s it a bit too quick and dirty. The only available transition is a fade, and the web preview has no transitions at all—as you click on something you get linked to a new, static, page. Especially since rough UI sketches tend to be difficult to parse, this is a major drawback. Without anything that resembles the native iOS slide animations you easily lose track of how the screens related to each other—which is, after all, the purpose of a prototype. It’s such a major drawback, in fact, that I wouldn’t recommend using the app in its current state. Hopefully this is something the woomoo guys will fix in a future update, but for now, FieldTest remains the best way to create iPhone prototypes from images.