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.
Zeplin - Collaboration app for UI designers & frontend developers
Framer - Bring your designs to life.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Flinto - The prototyping tool designers have been waiting for.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project