Platforma - Wireframe library of layouts and UI elements
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Bloo Lo-fi Wireframe Kit - Open source wireframe kit for design and prototyping
Form Wireframe Kit by InVision - A free wireframe kit from InVision
Apple Watch UI Kit - Quickly create Apple Watch app prototypes