Cascading Style Sheets. Check out the CSS Zen Garden and meyerweb for classic examples.
- Pure - a minimalist CSS toolkit.
- unsemantic - a percentage-based grid system.
- InK - SAPO's CSS framework.
- 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
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).