Arns Quick Reference - I'm a forgetful person, and I'm too cheap to buy an HTML Help book, so this is what I use to remember tags I need to remember.
Webmonkey - This website is full of HTML, CSS, PHP, Javascript, and other helpful tutorials and resources. Weekly articles about web programming are great.
Dynamic Drive - A very cool place to find some kind of Javascript that will make your website more interactive than you ever imagined.
Resource Index - A place to find pre-written PHP, and CGI scripts. These will truly make your website interactive like forums, blogs, news updaters, etc.