Extensions built with eCSStender greatly simplify the design process because you can author modern CSS using advanced selectors, properties such as border-radius, or custom font faces and rest assured that your design will work… even in IE6.
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.
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions. LESS runs on both the client-side (IE 6+, Webkit, Firefox) and server-side, with Node.js.