Justinmind have a nice design tool under the name Prototyper. It is possible to get Prototyper Free Edition and Prototyper Pro Edition.  Pro edition costs $495.   http://www.justinmind.com/ The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use. It is possible to download and install additional plug-ins (stencils) from here . http://pencil.evolus.vn/en-US/Home.aspx    Balsamiq is a small group of passionate individuals who believe work should be fun and that life's too short for bad software. This application is not for free. A Single User License – $79. Trail period 7 days.   http://www.balsamiq.com/ Other tools: Framer - Modern prototyping tool  Indigo Studio - Rapid, interactive prototyping  Mockingbird - Wireframes on the fly  Simulify - Interactive, shareable wireframes, mockups and prototypes  Solidify - Create clickable pr...