iAd Producer is a favorite of mine (I actually worked on its design for awhile at Apple) it’s a little-known but extremely powerful tool from Apple. Think of it as “advanced Keynote”, or “actually accessible Interface Builder”. Alas the app is meant for not-so-popular content like iAds and iBooks widgets, but it can easily be repurposed to prototype iOS and Mac apps. It handles UI elements, screen flow, and animation really well. Better yet, an iAd project is based in HTML5, CSS3, and javascript which a lot of designers are already familiar with.