Pencil Project, a free, open-source, cross-platform GUI prototyping tool, that people can easily install and use to create mock-ups in popular desktop platforms. Pencil is available as a standalone application and a Firefox extension. You can download it from here.
Pencil provides various built-in shapes collections by default, which includes flowchart elements, desktop/web UI elements, Android/iOS GUI shapes and general purpose shapes for drawing different types of user interface ranging from desktop to mobile platforms. There are also many other freely distributed collections on the internet provided by the community, which can easily installable to Pencil with simple drag-and-drop operation.
Pencil has pre-installed stencils for Android and iOS UI, which makes it even easier and faster to start prototyping apps with a simple installation. These stencils are included from version 2.0.2. Some of the stencil collections that you can try are collected in the download archive.
Drawing diagrams are as easy as UI design in pencil, because of its collection of flowchart shapes. Pencil also supports connectors used for “wire” shapes in the diagram.
Pencil provides a clipart browser tool integrated with OpenClipart.org to let users easily find cliparts and add them into the drawing by simply drag-and-drop. These cliparts are in vector format, which are easily scalable to appropriate sizes.
Pencil can export drawings into most of the popular formats including OpenOffice/LibreOffice, Image formats (JPG, GIF, PNG), text documents, Inkscape SVG, Adobe PDF and HTML page that can deliver to viewers.
For the web UI, Pencil provides links of an element to specific page in the same document. These links are converted to hyperlinks when the document is exported into HTML pages. This helps the user to understand the UI flow of an application mockups.