Kite is a native animation and prototyping design tool for the Mac. It features an intuitive timeline editor, WYSIWYG canvas, and a robust scripting interface. The iOS companion app, Kite Compositor for iOS, allows you to interact with your Kite designs right on your iPhone or iPad.
Incredible visual access to practically every aspect of Core AnimationCons:
Limited support for vector editing in-app
Kite Compositor is my new go-to prototyping tool. As a long time iOS developer, most of my prototyping was just done in Xcode because I had complete access to Core Animation. Kite, unlike other tools, is built on top of Core Animation and exposes all (or most) of its features directly. As a result, the animations it builds are almost guaranteed to look exactly the same on device.
Beyond that, it's just a well-designed app. As with most professional applications, the UI takes a bit of poking around at first to figure out how to do everything, but after just a day of playing with it, I became familiar with most of its capabilities.