What is a good online resource for css 'design patterns'?

The Floatutorial is a great starting point for learning the important CSS attribute "float" and how to use it to layout content using some common patterns including two-column and three-column liquid layouts.

Floatutorial takes you through the basics of floating elements such as images, drop caps, next and back buttons, image galleries, inline lists and multi-column layouts.

Some websites that address web design patterns are listed below. They do not specifically provide the HTML and/or CSS in order to achieve the desired results, but they do provide examples of live sites that you can view source on (or, even better, use Firebug).

UI-patterns

This is probably the best of the bunch. It breaks things down into categories that cover the breadth of web page design tasks. You'll find categories such as tag-clouds, live preview and user registration among many others. This is a really comprehensive resource that is well organized. It explains each pattern and provides plenty of examples.

Pattern Tap

Similar to UI-Patterns although currently not as comprehensive. It takes a more social approach to collating design patterns by allowing users to create their own categories ("user sets") and populate them with their own selection of sites.

Yahoo Design Pattern Library

Unlike the other two, this one doesn't provide many examples of real sites. It is well organized and quite comprehensive.

Elements of Design

This is a blog showcasing various elements of web design. It doesn't discuss the patterns, but is good as a quick source of inspiration, or as a means to start your own analysis.

I refer to A List Apart articles all the time for those sorts of things.

They do a lot of trial-and-error research to come up with really creative ways to handle those common CSS problems in the cleanest most portable way possible.