"Compass is a stylesheet authoring framework that makes your stylesheets and markup easier to build and maintain. With compass, you write your stylesheets in Sass instead of CSS. Using the power of Sass Mixins and the Compass community, you can apply battle-tested styles from frameworks like Blueprint to your stylesheets instead of your markup."
Ruby must be on the system in order for the style sheets to be compiled, but the output doesn't require it.
via @pengwynn. This is a smart use of Sass to define a separate style guide file for key presentation elements like fonts and colors that the designer can easily modify, while ensuring that the main style sheet can inherit all of those values.
This warms my front-end developer heart as I'm not fond of breaking up a site into separate style sheets for colors, spacing, fonts and the like as it becomes unmanageable or very cumbersome as everyone has to be aware of which files reference any given element, and should a significant change need to be made to the structure of the markup, it must be reflect throughout the site.