Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
React Proto - React application design and prototyping tool
Invision - Prototyping and collaboration for design teams
Balsamiq Mockups 3 - New public beta for the popular prototyping tool