As a developer I spend a lot of time with cool Firefox add-ons like Web Developer and Firebug. Today I found another great add-on which lets you create mock screens. It is called Pencil brought to you by The Pencil Project. Like all other add-ons it is freely available. Once you have installed it you are ready to go. Currently it comes with some common Window XP like widgets which are most commonly used in mock screens.

Mock screens produced by Pencil can be exported to png format. Quality of mocks produced is also superior than some other programs I have worked with. This is a great little tool for developers.