App Prototyping Tools For Designers

Principle makes it easy to create animated and interactive user interface designs. Whether you're designing the flow of a multi-screen app, or new interactions and animations […]

… a specialized remote screen viewing application intended as a tool to help designers create graphics for mobile applications, it has also proven to be useful for creating quick and dirty simulations, demos, and experience prototypes.

… makes it easy to quickly create high-quality custom controls for your iOS apps, Mac apps and for the web.

compile Compass/Sass easily without resorting to command line interface — Compass.app helps designers compile stylesheets easily without resorting to command line interface. It is written in Java and works in Windows/Linux/Mac OS X.