What are the best web development resources?

[ArthurKay]

I'm curious... what does everyone think are the best "free" online web development resources (other than this site, of course;) )?

More specifically, what websites will a new web developers really benefit from?

In my experience, I've found the following sites to be extremely useful. I think newer (front-end) developers can easily use them:
- W3Schools.com
- W3C HTML Validator
- Google Webmaster Tools

[NogDog]

For HTML and CSS, if you spend the time to read, digest, and understand the following, everything else will be icing on the cake:
http://www.w3.org/TR/html4/
http://www.w3.org/TR/CSS21/

For a lot of very good icing:
http://www.alistapart.com

Since my main area of work is server-side programming with PHP and MySQL, my main resources are:
http://www.php.net/manual/en/index.php
http://dev.mysql.com/doc/refman/5.0/en/index.html
http://phpbuilder.com/board/