Antetype is a design and layout software focused on visual design. It saves you a lot of time creating and optimizing UIs.

Balsamiq Mockups is a rapid wireframing tool that helps you Work Faster & Smarter.

Diagramo is an online flowchart software. It's FREE and Open Source ( GPL ) It's pure HTML5 Share and collaborate.

Lucidchart is a web-based application to draw diagrams.

Mock up an application and show what's important, the idea, the information, the interaction.

Pencil Project is an open-source GUI prototyping tool.

Axure RP is a wireframing, rapid prototyping, documentation and specification software tool aimed at web and desktop applications.

Cacoo is a user friendly online drawing tool. Multiple users editing the same diagram means Real-Time Collaboration.

Fluid UI is a browser-based wireframing and prototyping tool developed by Fluid Software and used to design mobile touch interfaces.

MockFlow is a collection of wireframe tools, prototyping tools, UI mockups, UX suite.

Moqups is a streamlined and intuitive web app that helps you create and collaborate on wireframes, mockups, diagrams and prototypes for any type of project.

Pidoco is a cloud-based collaboration software.