Pencil Project Portable is built for the purpose of providing a free and open-source GUI prototyping tool that people can easily install and use to create mockups in popular desktop platforms. It includes built-in shapes collections for popular mobile (Android and iPhone) and desktop (Windows and generic) UI designs as well as flowcharts and common shapes. Additional stencil collections and clipart can be easily added as well. Pencil supports connectors which can be used to "wire" shapes together in a diagram along. Pencil project also supports exporting to PNG, web page, PDF, SVG and Open Document formats.
What's new in this version:
Version 2.0.3 has fixed some bugs.