Epic Pxls - UI kits, icons, templates, mockups and more, free
iOS 12 GUI - Free collection of UI components and screens of iOS 12
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Invision - Prototyping and collaboration for design teams
Landing UI Kit - Handcrafted UI components for landing pages
NOW UI kit by InVision - A beautiful cross-platform UI kit (PSD & Sketch)