Here are a few resources you may find useful when it comes to designing and using css on your websites.
If you have any suggestions, please feel free to add them to the bottom of this thread as a reply and Ill update the list.
SitePoint CSS Reference
After recently coming out of beta status, the Site Point reference gives you detailed descriptions, working explanations and compatibility information to the entire CSS language.
When designing with CSS it is important to check that your stylesheet validates properly and is free from errors. An invalid stylesheet, can sometimes cause unexpected results to your sites layout. Always validate your stylesheet as part of your testing process.
CSS Layout Techniques
Glish offer a number of cross browser css based layouts that can easily be customised to fit your needs.
Using CSS and a simple list to create radically different list options
CSS Shorthand Guide
Using CSS shorthand can cut the size of your style sheets down and make them easier to read. This article gives you a short introduction to the use of shorthand.
CSS Tips & Tricks
Tips and tricks to better stylesheets.
Ten CSS tricks you may not know