Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.
CSSViewer - A simple CSS property viewer
WING - A phone carrier that is actually nice to you.
CSS Animation Cheat Sheet - Awesome CSS3 plugin for spiffy animations
Spectre.css - Lightweight, responsive and modern CSS framework for faster and extensible development.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.