Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Humaaans - Mix-&-match illustrations of humans with a design library.
Form Wireframe Kit by InVision - A free wireframe kit from InVision
Blush - Illustrations for everyone
iOS Wireframing UX Kit - An easy, speedy way to wireframe for iOS
Open Peeps - A hand-drawn illustration library.