Apple Watch UI Kit - Quickly create Apple Watch app prototypes
Ouch! Illustrations by Icons8 - Professional, perfectly matching, and customizable illustrations for any designs
Module 02: Layouts - Professional toolkit for UI/UX design
Control Illustrations - 108 free flat illustrations with customizable characters
MockMagic - A free, easy-to-use device mockup generator
Humaaans - Mix-&-match illustrations of humans with a design library.