Chris Eppstein, one of the Sass developers and creator of Compass, has written an excellent article on refactoring the digg.com stylesheet using Sass:
http://chriseppstein.github.com/blog/2010/05/25/refactor-my-stylesheets-digg-edition/
It demonstrates how an already well crafter stylesheet can be very much improved in several ways. Definitely a must-read!
Chris also has a bunch of other great articles addressing not just the how of Sass and CSS generally, but also the why: