ProjectHuddle - Get design feedback and client approval using WordPress.
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.
Savahapp - The all-in-one design sharing, collaboration, workflow and app prototyping platform
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Specfox - A free platform for creating beautiful website and mobile app specification documents for your team. No more word templates!
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project