yEd is a free general-purpose diagramming program with a multi-document interface.

Gephi is the leading visualization and exploration software for all kinds of graphs and networks.

Macaw provides the same flexibility as your favorite image editor but also writes semantic HTML and remarkably succinct CSS.

Lucidpress is a web-based desktop publishing software application.

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

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