Pencil is a GUI prototyping tool that can be used to create mockups.

Top features:
* Built-in stencils for diagraming and prototyping
* Multi-page document with background page
* On-screen text editing with rich-text supports
* PNG rasterizing
* Undo/redo supports
* Installing user-defined stencils
* Standard drawing operations: aligning, z-ordering, scaling, rotating...
* Cross-platforms
* Adding external objects

This is a fork of the dead original project at http://pencil.evolus.vn/