Seriously Helpful Links:

Emil Stenström has written a Beginners guide to CSS and standards.

Eric Costello's CSS Layout Techniques: for Fun and Profit concentrates on complex layouts and also explains how to work around browser bugs. 

An Introduction to Cascading Style Sheets by Norman Walsh, originally published in the World Wide Web Journal.

An article by Christopher Schmitt at Web Reference titled “Making Headlines with Cascading Style Sheets” shows you how to spice up Web page headers with a dash of CSS.  Style sheets can take the language of a document into account (with the ':lang' pseudo-class)… but only if the document is correctly labeled. W3C's I18N WG published the latest guidelines on Choosing a language tag.

Jens Meiert maintains a list of CSS properties, including those in working drafts, with links to the relevant specifications.

SitePoint offers a CSS reference (CSS level 2)

xhtml.com publishes an alphabetical CSS 2.1 reference with screenshots for most properties. 

HTML.net offers a CSS tutorial (also in several other languages). 

Stu Nicholls offers CSSplay, a site with CSS examples, including many uses of ':hover'.