Useful links

http://www.w3.org/
http://www.w3schools.com/ (covers HTML/CSS/JavaScript/ASP/XML/PHP)

HTML validators

http://online.htmlvalidator.com/php/onlinevallite.php (far better than the w3 one, I can also recommend the download version)
http://validator.w3.org/

PHP

http://www.php.net/
http://www.codeignite.com/ (php framework)
http://www.cakephp.com/ (php framework)
http://www.zend.com/

Ruby On Rails

http://www.rubyonrails.org/

WAMP - Windows Apache, MySQL, PHP

This installs apache, mysql and php on windows in one easy to use installer.

http://www.wampserver.com/en/

MySQL

http://www.mysql.com/
http://dev.mysql.com/downloads/gui-tools/5.0.html (GUI tools for admin/accessing mysql)
http://www.phpmyadmin.com/ (web based tool for admin/accessing mysql)

ASP.NET

http://www.asp.net/
http://msdn.microsoft.com/vstudio/express/ (free version of visual studio for asp.net etc development)

Payments

http://www.paypal.com/
http://www.protx.co.uk/
http://www.worldpay.com/
http://www.netbanx.com/