W3C’s Cascading Style Sheets, level 2 CSS2 Specification - This is the source for what CSS is supposed to look like. However, it generally avoids talking about what is supported in the real-world.
EchoEcho.com’s CSS Tutorial - Covers the basics of CSS.
Dave Raggett’s Introduction to CSS - A short guide to using CSS on your web pages.
TopStyle - a CSS editor.
CSS Zen Garden, Resources - Pointers to various articles about CSS layout techniques.
From Table Hacks to CSS Layout: A Web Designer