• World Wide Web Consortium (W3C)
  • Recommendations

XHTML:

• W3C Markup Validation Service
• XHTML 1.1 Reference
• xhtml.org
• W3Schools

CSS:

• W3C CSS Validation Service
• CSS Reference
• W3Schools

PHP:

• PHP Reference Manual
• php.net

Ruby & Rails

• Ruby
• Core Ruby API
• Ruby Standard Library
• Ruby on Rails
• Rails API
  • ActiveRecord
  • ActionController
  • ActionView
• rubyforge.org

Other:

• AJAX Matters
• AjaxPatterns.org
• Web Wiz Guide - ASP Tutorials
• QuirksMode
• Web Wiz Guide
• webmonkey
• Web Developers Virtual Library
• WebPatterns
• GPlotter
• The Living Internet
• Dynamic Drive DHTML (the eyes came from here)