In fact, it doesn’t even have to cost you anything: Free utility Pencil contains everything you need to create serviceable mockups for websites, desktop applications, and mobile smartphone apps.

A mockup of Firefox's Australis theme as it might look on the conventional Windows desktop. The mobile Firefox user interface is already in close alignment with the new style.

Verge user Sputnik8 just went all out, with a detailed, comprehensive, and incredible-looking mockup of the Windows desktop with a full-on Metro skinning. Skip to main content.

It already has a working desktop, Edge browser, Start Menu, the Microsoft Store, and Notepad, with other features coming soon. It's only a mockup and not a real OS, but a neatly built one at that.