Pencil - GUI Prototyping Tool

The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use. 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. It supports Easy GUI Prototyping, Built-in Shape Collections, Diagram Drawing Support, Export to various formats etc.