quickMockup - Online tool for creating mockups in the browser.
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.
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Invision - Prototyping and collaboration for design teams
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project