Justinmind - Define awesome experiences through wireframes & prototypes.
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.
Live Sketchpad - Live collaboration app
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project