Please suggest learning resources, good reference material and tutorials here. Feel free to add and organise categories and content as appropriate.

General HTML

     HTML5

• http://diveintohtml5.org  by Mark Pilgrim 
• HTML5 Basics 
• http://www.html5rocks.com/ 
• HTML5 Playground/Testing Ground

CSS

•  http://www.w3schools.com/css/default.asp
• CSS Tutorial
•  http://www.css3.info/
• Information Page On CSS3
•  http://www.css3.com/
• CSS3 Reference  

PHP

• http://www.php.net/
• Official Page Of PHP
•  http://www.w3schools.com/php/default.asp
• PHP Tutorial
• http://php.resourceindex.com/
• PHP Sourcecode Website 

MySQL

• http://www.mysqltutorial.org/  
• Tutorial on MySQL
• http://www.mysql.com/
• Official MySQL page   

Usability

• add links here

Python / Django

• http://docs.djangoproject.com/en/dev/intro/tutorial01/ by The Django Project.

Ruby on Rails

Working with APIs

Web Security