Invision - Prototyping and collaboration for design teams
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
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.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
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
Zeplin - Collaboration app for UI designers & frontend developers