Tools
These utilities will make you more efficient and effective at CSS.Basics
Color
Couleur.io - create harmonizing color palettes
Friendly Color Palette Generator
Realtime Colors - visualize your colors and fonts on a real site
Development
Stylelint - linter to avoid errors and enforce conventions
PostCSS - add vendor prefixes to CSS rules using values from caniuse.com
Sass - Syntactically awesome style sheets
Font and Typography
Utopia - scale type without breakpoints
Type Scale - type tool for responsive typography