Pencil is a free and open-source GUI prototyping tool that is quick, easy and works across multiple 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. The list of built-in collections also 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 which are freely available on the Internet.
Pencil also supports any exports formats including PNG and HTML. When exporting to HTML you can even define links and behaviours, allowing you to 'walk' clients through the UI mockup with buttons which actually work! This makes it much easier to model to flow of a user interface.
Pencil also supports exporting documents into popular formats including OpenOffice/LibreOffice text documents, Inkscape SVG and Adobe PDF.