Balsamiq Mockups is a wireframing tool to help rid the world of bad software.

Concept.ly is a Web and Mobile mockups tool. Convert wireframes and designs into interactive Apps.

Use Flinto to create iOS and Android prototypes made from your existing screen designs.

Create professional-quality flowcharts, Org charts, UML diagrams and more with Gliffy.

Justinmind is a powerful wireframe software to define web and mobile applications.

Lucidchart is a flow chart maker & online diagram software.

Design and collaborate on user interface concepts for your apps and websites with MockFlow.

Mockingbird is an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.

Moqups is the most stunning HTML5 app for creating resolution-independent SVG mockups & wireframes for your next project.

OmniGraffle is for creating beautiful, precise graphics. Available for Mac and iPad.