Web Developer Code

WDG HTML 4.0 Reference
HTML 4.01 / XHTML 1.0 Reference
W3C HTML Validator
HTML cleaner
HTML color codes
HTML Character Entity List
HTML/XML/XSL Beautifier
HTML + CSS Editor
HTML Editor (real time)
HTML reference
HTML Cheat Sheet

CSS reference
CSS Cheat Sheet

Mozilla Developer Network - MDN (Web)
MDN Web API reference
MDN Event reference
MDN JavaScript
MDN JavaScript reference
Google Hosted Libs - JavaScript (open-source)

JSFiddle - live results (js, html, css)
JSFiddle - Joe's public fiddles
JSFiddle - Pseudo-random number generator
CodePen - online editor
CodeSandbox - online IDE
JavaScript & HTML DOM reference
JavaScript Cheat Sheet
Firefox Debugger
Chrome Debugger
Chrome Debugger - keys

jQuery API (core)
jQuery API (UI)
jQuery Tutorial - JavaScript library

DOM Browser
Form Borders
Form Info
Generate HTML Table
HTML Tree
List All Links
List Embedded Objects
List Frames
Resize to...
Table Borders