Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
MockFlow - A super easy wireframing tool with all the other tools you need in the product design process
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Axure - The most powerful way to plan, prototype and hand off to developers, all without code. Download a free trial and see why professionals choose Axure RP 9.
Invision - Prototyping and collaboration for design teams
UX-App - HTML5 all-in-one mockup & prototyping tool that exports completed interfaces to working HTML + Javascript