Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Platforma - Wireframe library of layouts and UI elements
Form Wireframe Kit by InVision - A free wireframe kit from InVision
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
iOS Wireframing UX Kit - An easy, speedy way to wireframe for iOS
Apple Watch UI Kit - Quickly create Apple Watch app prototypes