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.
Material Design Kit Android UI Kit contains interface components and 61 great screens from Android 7.0 Nougat, made with Material Design Guidelines.
Get UI Kit is a lightweight and modular front-end framework for developing fast and powerful web interfaces.