An open-source GUI prototyping tool that's available for ALL platforms.
Easy GUI Prototyping
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.
Built-in Shape Collections
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.
Diagram Drawing Support
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.
Exporting to Different Output Formats
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.
Elements in a drawing can be linked to a specific page in the same document. This helps user define the UI flow when creating application or website mockups.
Linkings defined in a document are converted into HTML hyper-links when the document is exported into web format. This process creates an interactive version of the mockup in which viewers can see a simulated flow when clicking on the UI elements.