Recently I've discovered the Pencil project for designing and mocking up GUI interfaces.I've found that it is quite excellent for mocking up interfaces and it's also multi-platform.
Sadly though it's a little lacking in the Icon department out of the box. I recently discovered the excellent Modern UI Icons project created by Austin Andrews. As Pencil uses SVG (scalable Vector Graphics) for it's controls and the Modern Metro UI icons back also uses SVG, I decided to convert the Modern Metro UI icons pack into something that Pencil can understand.
There are over 1000 metro like icons in the modern metro ui icon pack and it is licensed under a fairly nice cc based license. You can download the pencil icon pack here