A List Apart
An excellent resource, full of articles on the use of CSS to create elegant solutions to webdesign, and hacks for when there's no other way.
CSS Zengarden
In some ways the perfect demonstration of the power of CSS. Dozens of different site designs, submitted by visitors. The rule is that they all must use the same html file, the style is completely CSS driven.
Position Is Everything
An excellent Resource of quirks, browser differences and CSS design.
CSS Edge
Eric Meyer's exploration of the potential of CSS design. A beautiful site.