principles.design is an open source resource created to help you understand and create Design Principles.
A collection of articles, videos, and resources made by designers at Facebook.
Interfacer is a list of top quality free design resources for your next project. All the resources listed on the website are free for commercial use.
Mindful Design is a book designed to infuse your design process with the understanding and appreciation the human mind deserves, Mindful Design presents a responsible deep dive into the areas of cognitive psychology and neuroscience that can most improve design.
References.Design helps you to get freshest inspirations from all over the popular design communities, as well as import your personal collections. You can easily manage all your online and offline assets in one place.