The Pencil Project provides a free and open source tool for making diagrams and GUI prototyping.
As quoted from the website, it features
- Built-in stencils for diagraming and prototyping
- Multi-page document with background page
- On-screen text editing with rich-text supports
- PNG rasterizing
- Undo/redo supports
- Installing user-defined stencils
- Standard drawing operations: aligning, z-ordering, scaling, rotating…
- Adding external objects
It is released under GPL version 2 and is available for virtually all platforms that Firefox 3 can run. The first version of Pencil is tested against GNU/Linux 2.6 with GTK+, Windows XP and Windows Vista.