Free Scripts
Various programming languages...
This one has been around for a while! :) Lots of free javascript/ajax scripts.
"Hot Scripts is the net's largest PHP, CGI, Perl, JavaScript and ASP script collection and resource web portal."
Lots of free (and some commercial) scripts of all kinds, especially PHP, but also has ajax, Flash, and a few Ruby on Rails.
A great classic site that's been around since DHTML was ajax and the web didn't have a version number. Now it's become a great repository of tutorials and scripts for ajax and CSS.
Like the 53 CSS Techniques article, presents a categorized list of different toolkits and scripts for specific purposes. Very informative. Now if only they would give us some dang anchors on these things...
CSS tutorials, experiments, techniques, and tools. Been around for a while, and just keeps getting better!
Code Generators
Wizards that make code for you based on your input; mainly CSS-centric, such as Listamatic!
Menu builder for Stu Nicholls Vertical five deep menu will generate your code for you! Thanks Jerry Anzalone! :)
Code generator with a few styles to choose from
Source-Ordered content code generator (PIEfecta layout generator) for 1-3 column CSS layouts, offers many options...
Great tutorial site, also the home of the famous Listamatic navmenu code examples. Helpful floating and selector tutorials are a must read.
Tools
Misc. things that don't fit the other categories... Validators, calculators...
W3C's easy-to-use
HTML validation service, based on an SGML parser.