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.
Get UI Kit is a lightweight and modular front-end framework for developing fast and powerful web interfaces.
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.