xCSS is an open source lightweight framework that bases on CSS to empower a straight forward and object-oriented workflow when developing complex styles cascades. This framework is very useful to cut out development time by: having an intuitive overview of the overall CSS structure, using variables, re-using style existing cascades, nesting selector declarations, extending objects and many other handy features. xCSS seemly integrated into any existing workflow. An advantage is that the CSS overhead is getting reduced while your (X)HTML attributes remain semantic.
xCSS compiles on the fly (your natural CSS workflow remains the same) and it easily configurable (you can setup the file hierarchy and miscellaneous output settings in one single xCSS config file).