Pencil for Windows

Pencil is an open-source GUI prototyping tool. It is free and easy to use and works on multiple platforms.

Pencil has a variety of built-in shapes that can be used to draw different types user interfaces, from desktop to mobile. Pencil comes pre-installed with iOS and Android UI stencils starting at 2.0.2. It is easy to create apps by simply installing the stencils. The built-in collections include general-purpose shapes, flowcharts, desktop/web UI shapes and Android GUI shapes.

You can also find many other collections that have been created by the community and are available online.

Pencil supports all export formats, including HTML and PNG. You can even create links and behaviours when exporting to HTML. This allows you to 'walk' clients though the UI mockup, with buttons that actually work! This makes it easier to model the flow of a user interface.

Pencil also supports exporting documents into popular formats including OpenOffice/LibreOffice text documents, Inkscape SVG and Adobe PDF.