Checklist Design - The best UI and UX practices for production ready design.
Ouch! Illustrations by Icons8 - Professional, perfectly matching, and customizable illustrations for any designs
Design Principles - An open source repository of design principles and methods
Control Illustrations - 108 free flat illustrations with customizable characters
The modern web design process - An in-depth guide to the processes behind great websites.
Humaaans - Mix-&-match illustrations of humans with a design library.