Learning HTML/CSS

W3Schools Online Web Tutorials
Tizag.com
HTML Goodies
DHTML Goodies
Cool CSS tutorials, examples and codes
53 CSS-Techniques You Couldn’t Live Without

Learning PHP

PHP.net
PHP Manual
10 Advanced PHP Tips To Improve Your Programming
50 Excellent AJAX Tutorials

Learning MySQL

MySQL Developer's zone
SQl Performance Tips

Learning Drupal

Drupal.org
Drupal Coding standards
Writing Secure Code
Drupal API references
Drupal Dojo
Lullabot Podcast

Learning Javascript/JQuery

JQuery Tutorials
JQuery Plugins
51+ Best of jQuery Tutorials and Examples
40+ Excellent jQuery Tutorials

Others

Complete List of Best SEO-Tools