Pencil
Pencil 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.
Pencil provides various built-in shapes collection for drawing different types of user interface ranging from desktop to mobile platforms. Starting from 2.0.2, Pencil is shipped with Android and iOS UI stencils pre-installed. This makes it even easier to start protyping apps with a simple installation.
Popular drawing features are also implemented in Pencil to simplify the drawing operations.
Starting from 2.0.2 Pencil has even more shape collections included by default. The list of built-in collections now includes general-purpose shapes, flowchart elements, desktop/web UI shapes, Android and iOS GUI shapes.
There are also many other collections created by the community and are distributed freely on the Internet. You can easily grab a collection and install it into Pencil with a simple drag-and-drop operation.
Pencil now supports connectors which can be used to “wire” shapes together in a diagram. A collection of flowchart shapes are also available for drawing diagrams.
Pencil supports outputing the drawing document into different types of formats. You can have your drawing exported as a set of rasterized PNG files or as a web page that can be delivered to the viewers.
Pencil also supports exporting documents into popular formats including OpenOffice/LibreOffice text documents, Inkscape SVG and Adobe PDF.