Pencil Project
Open SourcePencil Project is a free and open-source GUI prototyping tool that helps designers and developers create wireframes and mockups easily. Available as a standalone application and a Firefox extension, it offers a wide range of built-in shapes and templates for various platforms.
Key Features
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Free and open source with no cost.
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Easy to learn and use interface.
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Extensive built-in library of UI shapes and stencils.