Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It's translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.
It is a bunch of cool, fun, and cross-browser animations for you to use in your projects. Great for emphasis, home pages, sliders, and general just-add-water-awesomeness.
Less is a CSS pre-processor, meaning that it extends the CSS language, adding features that allow variables, mixins, functions and many other techniques that allow you to make CSS that is more maintainable, themable and extendable.
PostCSS is a tool for transforming CSS with JS plugins. These plugins can support variables and mixins, transpile future CSS syntax, inline images, and more.
Stylus is a revolutionary new language, providing an efficient, dynamic, and expressive way to generate CSS. Supporting both an indented syntax and regular CSS style.