Inside Design by InVision offers free design resources including high-quality UI kits, icon packs, mockups and more.
Get UI Kit is a lightweight and modular front-end framework for developing fast and powerful web interfaces.
Apple Design Resources offer great UI kits and guidelines to help you design apps quickly and accurately using Sketch, Photoshop, and XD templates.
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.