HyperText Markup Language (HTML) [W3C]
Standards, specifications, and guidelines on how to use the HTML and XHTML markup languages. From the World Wide Web Consortium, an organization that proposes and maintains technical standards for the Web.
Complete index and cross-reference of HTML tags; part of an ongoing project to bring accurate, material to the Internet community.
W3Schools.com: HTML 4.01 / XHTML 1.0 Reference
Find the complete list of HTML 4.01 and XHTML 1.0 tags along with their usages arranged by functions like char formats, frames, tables, lists, etc.
Quick Reference Site, The
Offers quick reference cards in PDF format for HTML and programming as well as books and free software.