HTML - CSS:
The most important site for HTML, XML and CSS information... They're the Gatekeeper.
http://www.w3.org/
A great online tutorial with interactive examples. Covers HTML, CSS and XML...
http://www.w3schools.com/
PHP:
The most important site for PHP. Founding fathers...
http://www.php.net/
Need some good PHP, Perl, ASP and Java scripts scripts and tutorials?... Huge database.
http://hotscripts.com/
See also: Quick Start Tutorial
