Prototyping iPhone Apps on iPhone

BUILD FULLY-INTERACTIVE PROTOTYPES FROM YOUR SKETCHES - FAST.
When we started designing Protosketch we wanted to make it dead simple to build an interactive version of your drawings in a matter of seconds.

Very cool looking free app (with in-app purchases) from the UI Stencils people. If you're in the early stages of planning an app this could be a fun and handy way of making a quick interactive prototype. I'll be playing with it later in the week to see if it lives up to the promise of speed and simplicity.

Source: https://protosketch.uistencils.com