Here you can find a pretty complete list to the most helpful guides for coding and overall helpful sites for your site. Please reply with a link to what you think should be added, if not already up here: Site Resources / Tools
Code
http://www.w3schools.com/
All around great site for scripting / coding and anything web related.
Code
http://www.sitescore.org/
A great free site that will rate your site based on current web standards and report back.
Code
http://www.kayodeok.btinternet.co.uk/favorites/webdesign.htm
A ton of resources for coding. HTML (and variations)
Code
HTML - http://www.w3schools.com/html/default.asp
HTML - http://www.html.net/tutorials/html/
HTML - http://www.htmldog.com/guides/htmlbeginner/
HTML - http://www.htmldog.com/guides/htmlintermediate/
HTML - http://www.htmldog.com/guides/htmladvanced/
HTML - http://www.htmldog.com/reference/htmltags/
XHTML - http://www.w3schools.com/xhtml/default.asp
DHTML - http://www.w3schools.com/dhtml/default.asp
CSS
Code
http://www.w3schools.com/css/default.asp
http://www.html.net/tutorials/css/
http://www.htmldog.com/guides/cssbeginner/
http://www.htmldog.com/guides/cssintermediate/
http://www.htmldog.com/guides/cssadvanced/
http://www.htmldog.com/reference/cssproperties/
http://lesliefranke.com/files/reference/csscheatsheet.html
PHP
Code
http://www.w3schools.com/php/default.asp
http://www.php.net/manual/en/
http://devzone.zend.com/node/view/id/627
http://www.phpbuilder.com/columns/vaska20050505.php3
http://www.php.happycodings.com/index.html
JavaScript
Code
http://www.w3schools.com/js/default.asp
http://www.javascriptmall.com/learn/contents.htm
http://www.trainingtools.com/online/javascript/
http://www.a1javascripts.com/
http://www.acjavascripts.com
SQL
Code
http://www.w3schools.com/sql/default.asp
http://www.1keydata.com/sql/sql.html
ASP
Code
http://www.w3schools.com/asp/default.asp
http://www.asp101.com/
http://www.asp.happycodings.com/index.html
Error Checking on Your Site
Visual Basic
Code
http://www.xtremevbtalk.com