Internet Explorer has cool add-ons which make the job of website designers and developers much easier. Here is my list of 20+ excellent IE  add-ons that every web developer and designer should know about.

Internet Explorer Developer Toolbar
 Variety of tools for quickly creating, understanding, and troubleshooting Web pages.

IE Watch
Allows you to view and analyze HTTP/HTTPS headers, Cookies, GET queries and POST data.

IE Web Developer
Allows you to inspect and edit the live HTML DOM, evaluate expressions and display error messages, explore source code of Web page and monitor DHTML Event and HTTP Traffic.

IESpy
Allows one to inspect or manipulate the DOM of any IE Web browser control.

DebugBar
Brings new services to surfers and professionals. Surfers: zoom, direct Web search, e-mail page screenshots, and color picker. Developers: view HTML code, cookies, JavaScript, HTTP/HTTPS headers, and miscellaneous information.

Virtual Machine
Allows you to view java applets on Web pages.

Microsoft Fiddler
Logs all HTTP traffic between your computer and the Internet.

Tangram Xtml Designer
Visual designer for IE Band Object, activeX Control and .NET user control.

Http Watch
 HttpWatch shows you HTTP and HTTPS traffic from within IE allowing you to quickly debug, fix and optimize your Web site.

Embedded Web Browser
The package contains all the programmer need to extend the development of a Web browser.

CGToolbar
Must have tool for CG Artists, Animators, VFX and 3d professionals.

Site Studio 6
Build rich content Web sites, with no HTML skills required.

Haptek Player
 An ActiveX control and Netscape Navigator Plugin that allows any webpage or application (with ActiveX support) to include Haptek’s Autonomous characters.

Flash2X Flash Hunter
Save Flash movies from web pages.

iOpus iMacros
Check the same sites every day,data upload, online marketing and functional testing and regression testing Web sites:

Telerik RadToolBar
A flexible component for implementation of tool and button strips, needed in most web applications.

UltraEdit-32
Powerful Text, HEX, HTML, PHP and Programmer’s Editor.

Bytescout Post2Blog
 Freeware powerful blog editor for WordPress, Typepad, MovableType and other blogs.

Search Monster
Free Flash Web Directory & Internet Search Engine is the Flash-remoting Web directory instant content for you Web site.

Zend Studio
Encompasses all the development components necessary for the full PHP application.

DbaBar
Integrated toolbar enabling Oracle Database Administrators to browse their databases from within Internet Explorer within minutes after installation.

Explorer Toolbar Maker
lets you create your own Explorer bar from any HTML page, picture, Macromedia Flash file, or Microsoft Office document.