Sites That Provide Services & Tools:

www.validator.w3.org
Validate your website and make sure it meets industry standards, this helps improve your website usability and makes your website SEO friendly.

www.google.com/support/webmasters/
Google Web Master Help Area full of useful tips and tricks.

www.drupal.org
Open Source CMS - incredibly powerful CMS.

www.joomla.org
Open Source CMS.

www.wordpress.org
Free Blog publishing software.

www.blogger.com
Free automated weblog publishing tool.

www.paypal.com
Easy to implement Payment Gateway for ecommerce websites.

www.pbwiki.com
Set up your own Wiki using pbwiki software.

Sites That Provide Information & Ideas:

www.wikipedia.org
Free encyclopoedia built collaboratively using Wiki software.

www.marketingsherpa.com
Resource that publishes information about internet & integrated marketing.

www.sitepronews.com
Webmaster news, advice and opinions offered by SEO professionals.

www.stylegala.com
A publication about web design and standards.

www.fuel4arts.com
Global knowledge base for artists and designers with ideas and leading arts marketing practice internationally.

Sites That Provide Answers For The Technically Minded:

www.sourceforge.net
Site that provides links to many nifty web applications and answers to many problems experienced when developing a website.

www.php.net
PHP programming language website, including introductory tutorials.