The Pencil Project looks like a pretty interesting tool for sketching out and prototyping a site design. It's based on XUL engine in Firefox 3 and looks to be a pretty impressive little open source application. From the Pencil Project website:
The Pencil Project's 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
- PNG rasterizing
- Undo/redo supports
- Installing user-defined stencils
- Standard drawing operations: aligning, z-ordering, scaling, rotating...
- Adding external objects
- And much more...
Pencil will always be free as it is released under the 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.
Make sure to check out the quick tour/screenshots of the application.