Start a new project or give an old Bootstrap project a new look! Now UI Kit is a free Bootstrap 4 UI Kit, available in HTML or Sketch/PSD format.
Apple Design Resources offer great UI kits and guidelines to help you design apps quickly and accurately using Sketch, Photoshop, and XD templates.
Material Design Kit Android UI Kit contains interface components and 61 great screens from Android 7.0 Nougat, made with Material Design Guidelines.
Inside Design by InVision offers free design resources including high-quality UI kits, icon packs, mockups and more.