CSS Properties

CSS Validator
- http://jigsaw.w3.org/css-validator/
W3C's CSS Test Suite to check your browser
CSS Editors:
- Style Master: http://www.westciv.com/style_master/
- and many others
Opera, Chrome, Safari, Firefox and IE8 now all provide much better integral tool support

Zen garden
- http://www.csszengarden.com/
Simple Bits from Dan Cederholm
- http://www.simplebits.com/
CSS Bugs Site
- http://positioniseverything.net/
Browser Support Charts
- http://www.westciv.com/style_master/academy/browser_support/
Indexdot CSS Index: http://www.blooberry.com/indexdot/css/
A tutorial from Western Civilisation:
http://www.westciv.com/style_master/academy/css_tutorial/index.html
Eric Meyer's list of Resources: http://www.ericmeyeroncss.com/links/resources.html
CSS Chapter from glasshaus's Web Professional's Handbook
(local copy)
Stylin with CSS site: http://www.bbd.com/stylin/ Examples from the book
FOUC: Flash of Unstyled Content (IE Bug)
Image Replacement
Unicode
Colours
Negative margins