Interfacer - Collection of more than 200+ free design resources
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Facebook Design Resources - A collection of free resources made by designers at Facebook
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Freebiesbug - Collection of the best free web design resources.
Platforma - Wireframe library of layouts and UI elements