Name
Pencil

Long Description
Pencil is 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
* 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…
* Cross-platforms
* Adding external objects
* And much more…

介绍晚些再写吧, 太惊喜了. 强烈推荐. 看来有必要研究一下Firefox的Addon开发了. 需要安装的同学请到这里来.