Designing and creating wireframes for web, mobile and tablet always should begin on paper. The creativity that gets explored while taking time to sketch ideas with a pen and paper remains the best tool instead of fumbling with software and apps. POP, also known as Prototyping on Paper, stays true to that fundamental, but then encourages you take pictures of your wireframes to auto-adjust sketches once the creative process has been completed. The app then lets you create interactive prototypes out of your drawings to round out balancing careful sketching with tools to quickly develop storyboards to showcase a polished idea the app.
Download it for free at the App Store.