[h=3]LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions. LESS runs on both the client-side (Chrome, Safari, Firefox) and server-side, with Node.js and Rhino.[/h]
LESS is a CSS Pre processor There are many CSS Preprocessors available, like Less, Sass, Emastic, 960.gs, Compass, Unobtrusive, BluePrint etc.
Check out http://lesscss.org/ It looks really interesting but I haven't got to the bit about how it handless css hacks yet. I do like the idea of being able to define a palette of colours and then use their readable names. I'm not sure if its not just a way to add complexity to a relatively simple system though.