UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Invision - Prototyping and collaboration for design teams
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.
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
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.