This UI Kit contains a collection of interface components and 61 great screens from Android 7.0 Nougat, made with extra attention to the Material Design Guidelines.
Apple Design Resources offer great UI kits and guidelines to help you design apps quickly and accurately using Sketch, Photoshop, and XD templates.
Inside Design by InVision offers free design resources including high-quality UI kits, icon packs, mockups and more.
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.