Pencil is an Open Source GUI prototyping tool designed for multi-platform mockup creation. A Portable version is also available.

Pencil provides a variety of built-in shapes for drawing different types of user interfaces with support for desktop and mobile platforms (Android and iOS UI stencils pre-installed).

Several popular drawing features are also implemented in Pencil to simplify the drawing operations. Pencil features numerous shape collections included by default. These collections include general-purpose shapes, flowchart elements, desktop/web UI shapes, Android and iOS GUI shapes. You can also access many other community-created collections that are widely distributed on the Internet. Adding a collection to Pencil is simple and can be done via drag n' drop.

Pencil includes a clipart browser tool that can be integrated with OpenClipart.org, allowing you to quickly find clipart via keywords and add them to a drawing. Clipart examples listed by Pencil are in vector format, allowing easy scaling to appropriate sizes.

Pencil will be a useful organizational asset for any developer or regular user wanting to view semi-refined mockups of their in-process ideas to see what flows best.