Firefox Plugins:

These are essential firefox plugins for web developers:
Development

• Web Developer Toolbar

Debugging

• FireBug
• FireRainbow
• FireFinder
• FireDiff
• FireCookie
• CodeBurner
• JSONView
• Live HTTP Headers

Design

• IETab (windows only)

Security

• TamperData
• XSS Me
• FireGPG

Online Web Dev Tools

• Favicon Creation Utility

Validation:

• XHTML Validation by W3C
• XHTML Validation by Validome
• CSS W3C Validator

Code Beautifiers and Optimizers:

• CSS Beautifier
• JavaScript Compressor
• JS Minifier
• HTML Entity Encode/Decode

Reference Materials:

• W3Schools XHTML Tag List
• W3C Markup Guide
• W3C CSS Guide
• HTML Doctypes

Online Tutorials:

• W3Schools XHTML
• XHTML on HTMLQuick
• JavaScript Tutorial
• Karate Corners

Cheat Sheets:

• CSS: Printable Sheet by AddedBytes
• CSS: Leslie Franke’s Reference
• HTML Tags: Visibone
• HTML: Visibone Popups

JavaScript

• The Ultimate jQuery List
• Mini Ajax
• Directory of JavaScript Libraries
• JSBin: Debug JS Live
• Sizzle (CSS Selector Library)
• Processing JS (A JavaScript port of the Processing Language)

Web Development Blogs

• A List Apart
• Perishable Press
• Coding Horror

CSS Libraries

• YUI Reset
• CSS Reset
• Blueprint CSS

CSS Tweakage

• http://pxtoem.com/ does conversions between px/pt/em/percent
• http://www.suprb.com/apps/csstype/ allows you to input text then apply all sorts of typographic styles to it so that you can preview how it will look.

Development Environment

• RapidSVN
• Uniform Server

Misc

• WordPress CMS Theme

Comment with any resources you wish to contribute.