http://pencil.evolus.vn/en-US/Home.aspx
The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
Top features:
- Built-in stencils for diagraming and prototyping
- Multi-page document with background page
- Inter-page linkings!
- On-screen text editing with rich-text supports
- Exporting to HTML, PNG, Openoffice.org document, Word document and PDF.
- Undo/redo supports
- Installing user-defined stencils and templates
- Standard drawing operations: aligning, z-ordering, scaling, rotating...
- Cross-platforms
- Adding external objects
- Personal Collection
- Clipart Browser
- Object snapping
- Sketchy Stencil
- And much more...
相当赞的Web原型设计工具，插件很小巧，安装到Firefox 3.5+即可使用。