Balsamiq – Balsamiq Mockups is a wireframing tool to help rid the world of bad software.
Concept.ly – Concept.ly is a Web and Mobile mockups tool. Convert wireframes and designs into interactive Apps.
Moqups – Moqups is the most stunning HTML5 app for creating resolution-independent SVG mockups & wireframes for your next project.
Mockingbird – Mockingbird is an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.
Justiinmind – Justinmind is a powerful wireframe software to define web and mobile applications.