Tetrisly - Starter kit for design systems and wireframes builder (Sketch/Figma)
Android Nougat GUI - Free fully customizable Android UI Kit for Sketch
Material Design Icons - 750 Free open-source glyphs by Google
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Material UI - A CSS Framework and a Set of React Components that Implement Google's Material Design
Epic Pxls - UI kits, icons, templates, mockups and more, free