CSS Tutorials
CSS Tutorial - A pretty good CSS tutorial.
W3 Schools CSS Tutorial - This tutorial has a "try them" feature where you can set styles and see what they look like.
CSS Reference
CSS Property Index - A very good and complete CSS property reference
W3C's CSS Page - A page of interesting links
CSS2 Specification - The CSS specification
CSS Utilities
CSS Box Border - A clever little form to try out different CSS box properties
W3C CSS Validator - Validate your CSS style sheets
Advanced CSS Sites
CSS Zen Garden - An excellent demo of the power of CSS.
A List Apart - Zeldman provides excelelent advanced Web design advice.
CSS/edge - Eric Mayer's site for pushing the limits of CSS.