The Startup Way - The new book from Eric Ries, author of The Lean Startup
Ouch! Illustrations by Icons8 - Professional, perfectly matching, and customizable illustrations for any designs
UI Playbook - The documented collection of UI components
Control Illustrations - 108 free flat illustrations with customizable characters
50 UI Tips - Improve your UI/UX skills in an hour, for free
Humaaans - Mix-&-match illustrations of humans with a design library.