Here at Potluck we recently started using Framer.js to prototype and try out interaction ideas we come up with for our iOS app. Framer is a Javascript framework for prototyping interactions and animations for mobile and desktop apps. It’s a great alternative to Quartz Composer as it has a less steep learning curve, and prototypes easily run both on desktop and the iPhone. We found Framer to be the perfect step up from Keynote to make realistic, interactive prototypes.