Ben Di Maggio

URLs:
Good as a reference or an introduction to CSS, this site also has a useful "try it" feature to allow you to test CSS styles:
http://www.w3schools.com/css/default.asp

A reference to most CSS styles from MS:
http://msdn.microsoft.com/library/default.asp? url=/workshop/author/css/reference/attributes.asp

Writing efficient CSS
http://www.communitymx.com/content/article.cfm?cid=A43B828960590F55&print=true

Catch-all CSS tips list
http://www.mezzoblue.com/zengarden/resources/

Inspiration
http://www.csszengarden.com

Meaning of quirks / strict modes, browser compatibility chart
http://www.quirksmode.org/css/contents.html

A guide to CSS in Plone:
http://plone.org/documentation/tutorial/where-is-what/whereiswhat

Book:
DTML and CSS for the World Wide Web, 3rd ed.  Jason Cranford Teague, Peachpit Press

CSS and HTML display tools:
Each of these programs lets you easily see the structure of other websites, giving you ideas as well as answers to the all-important question, "How did they do that?"

Mouseover DOM Inspector
http://slayeroffice.com/?c=/content/tools/modi.html

Web developer extensions for Firefox
http://webdeveloper.mozdev.org

Aardvark
http://www.karmatics.com/aardvark/

CMS-specific tools related to CSS:
CSSManager for Plone
Lets you configure the base_properties in a configlet. You can choose colors by clicking on a palette. (Thanks to Nate Aune for the recommendation.)
http://opensource.arche.de/products/cssmanager

WebTools for Mambo
The WebTools Component plugs into Mambo, providing tools Like ASCII lists, Color Pickers, CSS generator and Info. (Thanks to Scott Samenfeld for the recommendation.)
http://mamboforge.net/projects/webtools/