Magical stuff. Less is Leaner css. Less extends css by adding: variables, mixins, operations and nested rules.
CSS Naked Day is a great initiative where you disable all css files on your site for a day to highlight the values of good design. Awesome. Was 9 April in 2008.
Take multiple images, combine them into one, then use CSS to position the larger image such that only the small portion of the original image is visible. Combine multiple images into a single HTTP request.
The WP CSS plugin combines multiple CSS files used by WordPress into one single, gzipped file for faster download, fewer connections.
Firebug is a powerful debugging tool for web development. It's primarily a Firefox extension with a lite version available for IE, Opera and Safari. It allows you to debug, inspect, edit on the fly, and much more.
Nerdtastic article on using meta tags and CSS to optimise site widths for mobile devices. Delves into a vastly deep amount of detail that explains why a simple principle will usually work. Useful.
Very handy CSS hack cheat sheet. Includes a whole load of IE5 / IE6 / IE7 stuff. Came in very useful for an all but IE7 selector. :)