Pencil might have discontinued, try finding Pencil alternatives
Pencil is an open-source GUI prototyping tool that's available for ALL platforms. Pencil is built for the purpose of providing a free and open-source GUI prototyping tool.
Developed by Pascal Naidon and Patrick Corrieri
Apps available for
Mac OS X
Pencil is an Open Source Product.
Pascal Naidon and Patrick Corrieri has developed Pencil