W3C - Cascading Style Sheets:Authoritative information from the W3C. Provides complete specifications, lists of CSS browsers and authoring tools, and information about learning CSS.
Code Style - CSS Style Guide: Includes a cross-platform font sampler, information on media stylesheets, FAQ, and a glossary.
CSS Naked Day: On April 5th, simply remove all CSS from your website, stripping it entirely of its design. Promoting web standards, plain and simple.
css-discuss Mailing List: Practical discussions of CSS and its use. High volume list.
Dive into Mark: CSS: Archive of postings to Mark Pilgrim's weblog.
Eric Meyer - CSS: Articles, tutorials, discussion and resources for CSS.
EServer TC Library: Cascading Style Sheets: A collection of dozens of rated, reviewed online resources for developers who work with cascading style sheets in web and information design.
The CSS Pointers Group: CSS tips and resources. Site for members of the newsgroup for discussing CSS problems and solutions.
Web Developer's Virtual Library - Cascading Style Sheets: Teaches how to use CSS to improve the look and structure of HTML pages. Links discuss how to use CSS to avoid table layouts, colors, fonts, and layers.
WebmasterWorld: CSS Forum: Discussions on layout, compatibility and trouble shooting.