Platforma - Wireframe library of layouts and UI elements
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Icons8 Lunacy - Free graphic design software with built-in resources. Fully compatible with Sketch and works offline
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Flinto - The prototyping tool designers have been waiting for.