What are some alternatives?
Briefs - Briefs is the essential app design tool built for a professional design workflow.
Creo - Creo is a modern design tool for Mac which combines design and development.
AppCooker - Design, mockup and prototype mobile applications with AppCooker. Discover the most powerful mockup engine ever created on the iPad. AppCooker and AppTaster deliver the full scope of the prototyping tools.
Bubble - Don't hire engineers! Bubble let's you build an app entirely visually
Fluid UI - Fluid UI helps you prototype Android, iPhone, iPad or Windows 8 mobile apps with our custom libraries, or upload your own images for pixel perfect mobile
Invision - Prototyping and collaboration for design teams