Web Development Tools

Great listing of web developers powertools by Brennan Stehling.
I'll starts with extension for firefox that comes very handy while web devlopment.

• Web Developer : Adds a menu and a toolbar with various web developer tools.
• Firebug: Allows you to edit debug, and monitor CSS, HTML, and JavaScript live in any web page.
  
• ColorZilla: Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.
• Live HTTP Headers

The list Web Development Tools for the Power Developer contains :

Validators

• HTML/XHTML Validator
• CSS Validator
• Feed Validator (RSS and Atom)

Javascript

• Prototype Javascript Framework
• Script.aculo.us
• Yahoo! JavaScript Developer Center
• Javascript Lint

Websites

• A List Apart
• Position Is Everything
• Quirks Mode
• WaSP
• CSS Zen Garden

Read More>> (also containing ASP.NET, Proxy tools)
And w3schools, a great place for finding tutorials realted to web development.

Updated(29 April 2007) : Tools for web-designers.

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