MahApps.Metro is a toolkit for creating Metro-styled WPF applications. It also includes some custom controls based on concepts from Windows Phone, Windows 8 and Window 10 (UWP) Apps. Furthermore it's an open source and therefore free to use
MahApps.Metro is a project that Paul Jenkins started back in 2011 as a simple way to bring a Metro-style user interface into your WPF application.
Since then it’s evolved and taken contributions (at last count there were 600+ forks) from various people in the community.