Cascading Style Sheets. Check out the CSS Zen Garden and meyerweb for live examples.
Resources:
- Blueprint - a CSS framework with sensible printing defaults and grid layout support
- CSS redundancy checker - requires Ruby, but seems pretty useful.
- cssutils - a Python library for parsing and manipulating CSS.
- Techniques, by category.
- Dynamic Drive CSS Library
- The CSS Playground - lots of fun to be had.
- PVII Dreamweaver extensions lots of DHTML and CSS pre-built extensions for Dreamweaver
- Drop Shadows
- Slants (heck, every CSS geek has a link to this, even if it's fiddly to use...)
- Lists and DHTML menus
- Tables
If you're looking for the CSS DVD algorithm, the DeCSS Gallery has several renditions of it (and full coverage of the folly that ensued when it was broken).