JavaScript

www.javascriptkit.com/jsref
This site has JavaScript reference documentation, and tutorials to help you understand how to use JavaScript.
There are also reference documents on the DOM (Document Object Model) & CSS (Cascading Style Sheets).
w3schools.com
A mixture of tutorial & reference information, with lots of examples.
The site also has information on the DOM, CSS, XML, AJAX, and a whole host of other web-related technology.
Sun Microsystems' Client-Side JavaScript Reference.
The ultimate JavaScript reference site.
The site (http://docs.sun.com/) also contains a wealth of other useful documentation on web-technologies & Sun products.
My JavaScript Coding Guide.
A Guide to Writing Error-Free, Portable and Re-Usable JavaScript.

HTML

HTML 5 Reference, from W3C
This is the reference that I use all the time.
There is also a reference of HTML Entities (special character codes).
HTML 4 Reference, from the Web Design Group
It is available in English, German & Japanese.
You can view HTML Elements alphabetically, or organisationally; either all, or just the strict list. There is example code, and links for the types of the various attributes.
There is also a reference of HTML Entities (special character codes).
w3schools.com
A comprehensive reference to all HTML tags.
The Official W3C Reference for HTML 4.01
This is the official HTML 4.01 specification. It is very comprehensive, but not designed for quick reference.
The W3C HTML Validator
You can use this page to validate the HTML in your web-pages.

Cascading Style Sheets (CSS)

The W3C CSS Home page
CSS specifications, tutorials, etc. The specifications are not that easy to navigate or use, but my CSS "bible" is hidden away on this site: http://www.w3.org/TR/REC-CSS2/propidx.html.
The W3C CSS Validator
You can use this web-page to validate the CSS content of your web-pages.
ZVON CSS 2 page
Tutorials, specifications, etc. Very comprehensive.
The Web Design Group's CSS page
Tutorials, specifications, etc. Very comprehensive.
w3schools.com
A comprehensive guide to CSS.
CSS Zen Garden
An interesting site showing the incredible things that can be achieved with Cascading Style Sheets. There are several different versions of the same site, all with different styles and layout, all achieved by using different style sheets (i.e. the HTML is the same for all versions).
MicroSoft's CSS Reference
The MicroSoft Web Developers Center CSS Reference.

DOM (Document Object Model)

MicroSoft DOM reference
One of the big challenges is always to make things work in both Internet Explorer and W3C compliant browsers like Mozilla. The MicroSoft reference is essential for this reason.
Beware: MicroSoft reorganises their web-site often, so the link may not work. It can be hard to find the DOM reference by navigating from the top of the site.
Gecko DOM Reference
Easy to use and well structured DOM Reference. It includes some example code. Largely compliant with the W3C standard.
Gecko is the rendering engine used in the Mozilla Firefox browser.
w3schools.com
Easy to navigate reference to the W3C (the standard) DOM standard.
The W3C DOM Standard
This is the industry standard, which everyone except MicroSoft tries to comply with.
Not very well structured, but it is the ultimate reference.

Java

The official Java API Reference
Java was created and is owned by Sun, so this is the official reference source.
The API is the list of all the Java Classes provided in the JRE (Java Run-time Environment). It lists the classes, how to instantiate them (when appropriate) and their methods (interfaces).
The official Java Tutorials
These are the Sun Microsystems' tutorials on Java programming.
Java Language Specification
The Java Standard Edition 6 Language Specification. It can be a little hard to find what you need, but everything is here.

Perl

The official CPAN web-site
A comprehensive source for Perl Modules: documentation and modules to download. Perl Modules are libraries that you use to do things such as generate HTML, connect to a database, etc.
The official Perl web-site
Contains the core documentation for Perl version 5.10.0, in HTML and PDF formats. If you want to know about the Perl language and data-types, this is where to look.

MySQL

The official MySQL site
MySQL is an Open-Source database, which is now owned by Sun Microsystems (and Sun is now owned by Oracle). This site has reference manuals for the current versions of MySQL. These manuals include a tutorial, which is a good way to get yourself started with SQL and databases.

Oracle

The official Oracle site

Oracle is the number one database for large complex business critical applications. It is used in telecoms, finance and a whole host of other business domains.

Oracle also owns Siebel (a very successful CRM product) and Sun Microsystems.
OraclePool.com
This is a good site to get an introduction to the Oracle database. It is owned and created by my friend Hareesh.