Cascading Style Sheets: latest news from W3.org
CSS Play: Experiments with CSS: layouts and demos
CSS Zen Garden: submit or use creative commons designs
Hands On CSS: free layouts and resources
Learn CSS Layout: very clear and helpful layout hints
Mozilla Developers' CSS Reference: reference and links to tutorials
Accessibility Tutorial: W3.org's overview
Anybrowser Pages: links to sites & information concerned with cross-browser compatibility
CSS Validation Service: tool provided by W3.org
US Web Design Systems: Accessibility: official US government standards
Cnet: Downloads: free software
ColorHexa: color information & a matching color generator
ColorSchemer: easy way to get harmonized colors
HTML Color Picker: from W3Schools
Classroom Clipart: large graphic archive for students & teachers
Clker.com: public domain clip art
Creative Commons: search collections of royalty-free images
Dynamicdrive: source of free galleries, generators, scripts, etc., plus demonstrations & tutorials
IconArchive: graphics archive searchable by subject
Morguefile.com: collection of freely-usable photos
Openclipart.com: free images
Photoeverywhere: Creative Commons collection of photos from around the world (especially UK)
Pixabay: very high-quality photos free to use through Creative Commons
Realm Graphics: one of my favorite graphic sites for icons
Unsplash: quirky Creative Commons graphics
Color Schemer: easy way to get harmonized colors
Dynamicdrive: source of free generators, scripts, etc., plus demonstrations & tutorials
HTML for Beginners: tutorials and a forum
HTML Entities Reference: complete basic set of character entities from W3 Schools
HTMLGoodies: articles & forums on a variety of subjects
mjdwebdesign: instructional videos channel
Quackit Tutorials: tutorials & references on a variety of subjects
Tizag Tutorials: tutorials and references
W3Schools: tutorials & references from the most official source
Web Development in the Classroom: Web Site Building for Kids: collection of links to tutorials and websites to help students and teachers with web page production
Web Developer's Handbook: voluminous tutorials, references and tools
Web Design Group: articles are dated, but still has lots of good information
WebmasterWorld: forum on web design topics