I first noticed Pencil on Freewarefiles.com…
Pencil is a free drawing program that helps users to create mockups on their PCs. Its friendly graphic user interface provides ready access to a collection of built-in shapes that can be used to create interfaces for both mobile and desktop platforms. The prototyping tool is available for all platforms.
If you want a powerful but free drawing program, then Pencil is a good choice. Even beginners will find it relatively easy to use because it works on the “What You See Is What You Get” (WYSIWYG) basis.
SOURCE: Pencil Project