Facebook - Connect with friends, family and other people you know. Share photos and videos, send messages and get updates.
Design Principles - An open source repository of design principles and methods
Straple - An end to end design system in Sketch and ReactJS
Facebook Design Resources - A collection of free resources made by designers at Facebook
Angle 2 Mockups - A giant Sketch Library for creating app presentations
Atlassian Design - Design, develop, and deliver