Pencil is an Open Source GUI prototyping tool designed for multi-platform mockup creation.
Pencil provides a variety of built-in shapes for drawing different types of user interfaces with support for desktop as well as 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 additionally access many other community-created collections by the 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 into 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.