CSS 2.1 Specification (W3C)

http://www.w3.org/TR/CSS21/

The definitive specifications.

CSS Play

 http://cssplay.co.uk

 

CSS Zen Garden

http://csszengarden.com 

A site that shows the incredible power of CSS!

CSS Creator

http://csscreator.com

 A site dedicated to CSS.

Real World Style

 http://realworldstyle.com/

Dynamic Drive CSS Layouts

http://www.dynamicdrive.com/style/layouts/

This site provides a very useful set of templates

On-line Code Manual