Android Nougat GUI - Free fully customizable Android UI Kit for Sketch
Module 02: Layouts - Professional toolkit for UI/UX design
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Epic Pxls - UI kits, icons, templates, mockups and more, free
Apple Watch UI Kit - Quickly create Apple Watch app prototypes