Sass
I’m very interested in preprocessors, both JavaScript and CSS based. They provide improved organization through logical code separation and time savings through the use of mixins, variables, and functions. Sass is a great Ruby based CSS preprocessor. Compass is an extension of Sass which adds even more power, including the addition of reusable patterns (easily import common css normalizers, boilerplates, etc) and one of my favorite features, automated sprite image and css generation. Stylesheets have never been so powerful.