Compass and Twitter Bootstrap are two excellent web development frameworks built on top of CSS pre-processors (Sass and LESS respectively). Both can be very useful even though the approaches they take are rather different.
Sass is a language / framework which compiles down to CSS. The thing I like most about Sass is how it helps you apply the DRY principle to your style sheets. If you use it well, you can drastically cut down “copy-and-paste” code and actually get reasonably maintainable CSS to work with.