Cacoo is a user friendly online drawing tool that allows you to create a variety of diagrams such as site maps, wire frames, UML and network charts. Cacoo can be used free of charge.
The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
Need a diagram, process chart, quick page-layout, website wireframe or graphic design? OmniGraffle can help you make eye-popping graphic documents quickly.