Haiku - Haiku is an open source OS catered specifically to the needs of personal computing.
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.
Creo - Creo is a modern design tool for Mac which combines design and development.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Flinto - The prototyping tool designers have been waiting for.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project