The Pencil Projects unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
Built-in stencils for diagraming and prototyping
Multi-page document with background page
On-screen text editing with rich-text supports
Exporting to HTML, PNG, Openoffice.org document, Word document and PDF.
Installing user-defined stencils and templates
Standard drawing operations: aligning, z-ordering, scaling, rotating...
Adding external objects
And much more...
Pencil can be installed as Firefox extension or as a standalone application for Windows, GNU/Linux and MacOS X.