Inside Design by InVision offers free design resources including high-quality UI kits, icon packs, mockups and more.
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.
Shards is a free and modern UI toolkit for web makers based on the popular Bootstrap 4 framework. It’s lightweight, responsive and well documented.
Apple Design Resources offer great UI kits and guidelines to help you design apps quickly and accurately using Sketch, Photoshop, and XD templates.