CSScaffold is a script written in PHP that parses your CSS files transparently (by using a .htaccess file), to extend the capabilities of vanilla css.
It lets you use nested selectors, constants, expressions and mixins all while using the standard CSS syntax.
It has an easy-to-use plugin system, so you can add your own features (you might want to set constants via xml so a CMS can integrate with your CSS).
Get the latest Scaffold here - github.com/anthonyshort/csscaffold/tree/master
Documentation - wiki.github.com/anthonyshort/csscaffold