purplemass.com

This site beautifully demonstrates how easily/quickly style sheets can change the appearance of a web site:

http://www.csszengarden.com/

Simple to follow and very informative:

http://hotdesign.com/seybold/everything.html

This one looks pretty useful:

http://www.dissurion.com/category/css/

Best reference site out there:

http://www.w3schools.com/css/css_reference.asp

Top 5 CSS layout tips:

http://ventureskills.wordpress.com/2007/05/09/top-5-css-layout-tips/

Some more advanced css:

http://www.cssplay.co.uk/menus/simple_vertical.html
http://www.projectseven.com/tutorials/css/uberlinks/
http://www.maxdesign.com.au/presentation/center/

These links show how style sheets behave differently between various browsers and why:

How many times have you heard someone say: “Why does my site look different in IE than in Firefox”?

Really Undoing html.css

Universal Selector

Undoing html.css and using debug scaffolding

Global White Space Reset