PostCSS - Increase code readability. Add vendor prefixes to CSS rules using values from Can I Use. Autoprefixer will use the data based on current browser popularity and property support to apply prefixes for you.
TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.
Sass - Syntatically Awesome Style Sheets
Free Texture Packer - Free Texture Packer is an open source tool that allows you to pack multiple images into one atlas for you games or sites.
Stylus - EXPRESSIVE, DYNAMIC, ROBUST CSS
Sprite Sheet Packer - Sprite Sheet Packer is online that you can easily use to pack sprite sheets, including data formats for CSS, LESS, and Phaser.