Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
iOS Design Kit - The newest library of native iOS templates
Landing UI Kit - Handcrafted UI components for landing pages
iOS 12 GUI - Free collection of UI components and screens of iOS 12
Epic Pxls - UI kits, icons, templates, mockups and more, free
Ionic Sketch UI Kit - Smart Sketch library for Ionic 3