Humaaans - Mix-&-match illustrations of humans with a design library.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Control Illustrations - 108 free flat illustrations with customizable characters
Form Wireframe Kit by InVision - A free wireframe kit from InVision
Ouch! Illustrations by Icons8 - Professional, perfectly matching, and customizable illustrations for any designs
iOS Wireframing UX Kit - An easy, speedy way to wireframe for iOS