App Layout is a collection of Polymer elements, along with guidelines and templates that can be used to structure your app’s layout. It's used by most Polymer progressive web apps.
Early this year, I was able to contribute to this project by fixing some minor issues with a demo template. As a Polymer fan, this was pretty exciting for me. Currently, this project only has 18 contributors. Polymer 2 is right around the corner, I hope to contribute more and I hope you join us.