Interfacer - Collection of more than 200+ free design resources
The modern web design process - An in-depth guide to the processes behind great websites.
Facebook Design Resources - A collection of free resources made by designers at Facebook
Another Lens - A research tool for conscientious creatives by Airbnb
Freebiesbug - Collection of the best free web design resources.
Design Principles - An open source repository of design principles and methods