Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Android Nougat GUI - Free fully customizable Android UI Kit for Sketch
Platforma - Wireframe library of layouts and UI elements
Craftwork - A collection of User Interface resources made by Craftwork
Epic Pxls - UI kits, icons, templates, mockups and more, free
Invision - Prototyping and collaboration for design teams