Internet Explorer Developer Toolbar (freeware)
The Internet Explorer Developer Toolbar add-on for Internet Explorer is an essential tool for web developers. This toolbar enables you to explore and modify the document object model for a webpage, find a specific element on a webpage, view HTML object class names, IDs and other details, and validate HTML and CSS.
This toolbar, however, is not compatible with Internet Explorer 8. It has been replaced by Developer Tools included in Internet Explorer 8.
DebugBar (commercial, free for personal use)
Web Development Helper (freeware)
The Web Development Helper add-on for Internet Explorer is a set of tools for web developers. It has a DOM inspector, an HTTP tracer, and Script console among other things. This tool is particularly helpful for the ASP.Net coder with features specific to ASP.Net development.
HTTPwatch (commercial, basic version free)
HTTPwatch is an HTTP viewer and debugger add-on for Internet Explorer and is also available for Firefox. It also has support for HTTPS and compression.
Web Accessibility Toolbar (freeware)
Web Accessibility Toolbar allows developers to assess and analyze the accessibility of a web page. It examines a web page for various aspects of accessibility.
IEWatch (commercial, free to try)
IEWatch is a powerful add-on for Internet Explorer for web developers. Its features include:
HTTP Analysis: It allows viewing HTTP GET query strings, POST data and cookies. It can also display unencrypted HTTPS data.
Filtering HTTP Data: It allows for the filtering of HTTP requests extensively to make it easier for the developer to find what they are actually looking for instead of being flooded by a huge volume of HTTP requests.
HTML analysis: It also has a HTML Analysis mode which provides an overview of the HTML elements that are embedded in the page.
IEWatch Tools: It has a mixed set of tools which enables many features like capturing a screenshot of a webpage, apply transparency to the Internet Explorer window, and resizing the IE window.
IEWatch also has a scripting interface which extends the Internet Explorer DHTML object model. This interface allows developers to integrate IEWatch with any testing framework.
Useful IE7 and Firefox Browser Extensions and Add-ons
Add-Ons for Microsoft Internet Explorer 7 users