Modern UI Icons provides Windows 8 Modern UI or Windows Phone style icons. License is under CC (Creative Commons), and can be used for commercial, and can make modifications. I use icons based on this resource for my Android app, design tool for Animal Crossing, New Leaf.
![Windows 8 / Phone Style Free Icons Windows 8 / Phone Style Free Icons](https://programresource.net/images/2013/05/modernuiicons-120x120.png)