Lampdevelopers offers a list of almost tools, reference sites and widgets for web developers. These are:

Sherlock is a development tool for examining, validating, and debugging web pages using Internet Explorer (it can also be used in FireFox or other browsers by importing the Sherlock folder and bookmarks).

Sherlock is a collection of the best developer bookmarklets from around the web. Each bookmark let performs a certain task on the current web page in the browser.

All Things CSS: Firebug 1.0 - Web developer tool of the year
A mandatory download for all Web professionals - it is packed with features so unbelievably useful you won’t know how you’ve been able to live without them. I add: WOW! Love it!

Web Design: Firebug 1.0 - Web developer tool of the year Not rated yet.
A mandatory download for all Web professionals - it is packed with features so unbelievably useful you won’t know how you’ve been able to live without them.

Web Development Firebug 1.0 - Web developer tool of the year. A mandatory download for all Web professionals - it is packed with features so unbelievably useful you won’t know how you’ve been able to live without them.

The Firefox web browser is fast becoming known as an excellent browser, but it is not as well recognized as a great web developer tool. Though not an exhaustive list, the following Firefox extensions allow a web developer to more easily create and manage a web site. Obviously some extensions will be more valuable to some developers than to others, but the Aardvark, Colorzilla, and Web Developer extensions should be considered an essential part of any developer’s toolbox.

Aardvark is an excellent CSS debugger with a bunch of keyboard modifiers. Shows things such as the element name and id or class name of a selected element.

Colorzilla is an eyedropper tool with several other useful functions. Hovering over an element will show color info, DOM path info, and X, Y offset from the previous click. In addition, Colorzilla provides a magnifying tool and a color mixer.

Fangs presents a textual representation of a web page, similar to the way it would be read by a screen reader.

Fireftp is a FTP client which allows for easy access to a FTP server.
Link Checker checks every link on a web page to find bad links. Link Checker’s simple color coding scheme makes it easy to spot a bad link, quickly, on any page.

Live HTTP Headers displays information about the HTTP headers on a page.