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WireframeSketcher is not open-source but it's free for open-source developers. WireframeSketcher helps you quickly create wireframes, mockups and prototypes for desktop, web and mobile applications. It comes both as a standalone version and as a plug-in for Eclipse IDEs. It has some distinctive features like storyboards, components, linking and vector PDF export. Among supported IDEs are are Aptana, Flash Builder, Zend Studio and Rational Application Developer.

(source: wireframesketcher.com)