The root of all things WWW is the W3C: http://w3.org/

The Open Web Application Security Project (OWASP) is an Open Source community project developing software tools and knowledge based documentation that helps people secure web applications and web services. At this point it looks as if their most important document is The OWASP Guide to Building Secure Web Applications and Web Services.

http://www.w3.org/2001/tag/doc/whenToUseGet.html - "URIs, Addressability, and the use of HTTP GET and POST", i.e. when to use GET and when to use POST.

JavaScript

I'm not sure that I like javascript, but it looks as if a good approach is to use a library which hides browser incompatibility. Like RelativeLayers http://relativelayers.sourceforge.net/

A guy at work likes these sites for javascript: http://www.javascriptkit.com/ (used to be called website abstraction) and http://www.js-examples.com.

http://sourceforge.net/projects/jscalendar - very nice popup JS calendar, works only with modern browsers (IE 6/Moz). LGPL.
http://mojavelinux.com/forum/viewtopic.php?t=6 - Javascript popup calendar for date input on web pages. Claims to be work with many browsers.

http://kryogenix.org/code/browser/ - some interesting code such as tables that can be sorted by clicking on their headings.

OpenThought - This project uses an interesting twist on the standard web paradigm: it downloads a page once and then uses javascript and DHTML to stream changes into the page. You never have to refresh the page. Cute. http://www.openthought.net/
Here's an article about a similar approach: http://www-106.ibm.com/developerworks/web/library/wa-httpget/index.html

Here's an article that uses applets as proxies for javascript. It's an interesting approach that keeps the applets small but allows you to use database data to refresh web pages.
Here's an all-JS implementation: http://oss.metaparadigm.com/jsonrpc/

AJAX

Async JS and XML.

http://prototype.conio.net/ - seems to be the ur-library for AJAX. Has no documentation.

Docs for Prototype:
http://www.sergiopereira.com/articles/prototype.js.html
http://blogs.ebusiness-apps.com/jordan/pages/Prototype%20Library%20Info.htm

http://www.xml.com/pub/a/2006/08/23/xsldatagrid-xslt-ajax.html - the XSL data grid - send plain XHTML to the browser, then XSLT transform it, then plug it into the DOM.

ASP

http://www.tripi.com/arrowhead/ - this is a Java Servlet application that supports simple VBScript ASP syntax. Might be useful if I ever wanted to move an ASP application into a more open environment.

http://asp2php.naken.cc/ - an ASP/VBScript to PHP converter.

CSS

http://www.meyerweb.com/eric/css/edge/index.html - advanced css positioning
http://style.tigris.org/ - a set of open-source CSS stylesheets. A really cool idea, but unfortunately unmaintained
http://www.mollio.org/ - similar to style, a set of open-source CSS stylesheets. Very good-looking.
http://gallery.theopalgroup.com/selectoracle/ - a tool that parses and explains CSS selectors.
http://csszengarden.com/ - an amazing example of what CSS can do
http://css-discuss.incutio.com/?page=UsingPercent - a page that shows how to use CSS to make images resize dynamically when the page size changes
http://www.gamingheadlines.co.uk/wod/formstyle/index.html - how to use CSS and JS together to add style to checkboxes and radio controls

Certificates

http://www.cacert.org/ - a community-based certificate provider
http://www.xrampssl.com/ - an alternative digital certificate provider.
http://www.freessl.com - a low-cost commercial certificate provider.

Fast Web Services - when XML costs too much - http://developer.java.sun.com/developer/technicalArticles/WebServices/fastWS/

Browser Caching - Internet Explorer has badly broken cache behavior, but this appnote can help you work around it: http://support.microsoft.com/default.aspx?scid=http://support.microsoft.com:80/support/kb/articles/Q234/0/67.ASP&NoWebContent=1&NoWebContent=1

Opening Pop-ups - this a hugely bad idea, but sometimes the client insists. http://www.sitepoint.com/article/1041/1

http://squidfingers.com/patterns/ - a set of tiled patterns, good for backgrounds.

http://www.mozilla.org/docs/web-developer/quirks/ - explains the different ways that Mozilla renders web pages and how to choose which one gets used.

Presentations - http://meyerweb.com/eric/tools/s5/