Pencil Project

Pencil project is a unique and cool wireframe builder out there. The best part is, it is not only downloadable for Mac and Windows, but it can also be added as a Firefox add-on. Pencil Project is an amazing free wireframe design tool with lots of templates to choose from for mobile app designing and web designing. This all-in-one wireframe builder is known for its diagramming capabilities. Pencil Project comes with a "connector" tool which makes sure that linking wireflows, flowchart and shapes becomes seamless.

The best part is Pencil Project is completely free to use.