Collect, test, and experiment with interface pattern pairings of CSS & HTML. Pears is an open source WordPress theme, enabling people like you to get your own pattern library up and running quickly.

PageBlox helps web designers build rock-solid HTML/CSS templates in seconds, not hours.
Harness the power of the blueprint.css framework and grids to generate cross-browser compatible code.
Slice and dice divs, adjust div/column/gutter widths without touching a line of code.
Generated code is fully supported by all major browsers.

Bootstrap is a toolkit from Twitter designed to kickstart development of webapps and sites.
It includes base CSS and HTML for typography, forms, buttons, tables, grids, navigation, and more.

CSS Tabs with submenus; also includes links to other "CSS tab systems"

Website on CSS

CSS-based way of displaying banners on a website in a better way

How to have nicely aligned custom bullets with CSS

CSS-styles footer at either the bottom of the browser window or the bottom of the page, whichever is lowest

"CleverCSS is a small markup language for CSS inspired by Python that can be used to build a style sheet in a clean and structured way. In many ways it's cleaner and more powerful than CSS2 is.
The most obvious difference to CSS is the syntax: it is indentation based and not flat. While this is obviously against the Python Zen, it's nonetheless a good idea for structural styles."

How to embed different CSS stylesheets for different (buggy) browsers

Overview of various table layouts, including some very nice and beautiful ones.

How to make semantically correct, good-looking HTML forms

"css-discuss is a mailing list devoted to talking about CSS and ways to use it in the real world; in other words, practical uses and applications."