What is Fiddler?
Fiddler is an HTTP Debugging Proxy by Bayden Systems (Fiddler 2.2 and below). There is also a version from Microsoft (Fiddler 188.8.131.52) and an updated version (Fiddler 184.108.40.206) from Eric Lawrence of Microsoft.
A Web Debugger freeware program that is used for tagging HTTP requests. In fact, it sees all HTTP sessions, requests, and responses before being returned to the client application. (Note: For applications to run with no user interface, FiddlerCore (a version of Fiddler by Microsoft) will run applications as a library.)
Current version is Fiddler 2.x (File size: 658 KB) by Eric Lawrence. Unlike the previous version, Fiddler 2 supports debugging HTTPS traffic. "By default, Fiddler2 accepts SSLv2 SSLv3 and TLSv1 from the client, and offers SSLv3 and TLSv1 to the server." 
How it works:
- It works automatically with Internet Explorer, but any browser or application can be configured to route traffic through Fiddler.
- It uses the proxy address 127.0.0.1 and port 8888.
- Every Web request that is routed via the proxy is seen as a separate node in the web sessions list.
- It inspects all HTTP Traffic that is incoming or outgoing data.
- It can log and profile HTTP/HTTPS traffic that has been sent, received, and redirected over the network.
- It can be used as a shared proxy server running on one PC that can store a full registry of all the HTTP traffic.
- It can be configured to run in reverse proxy.
- It is able to eliminate specific scripts that slow down system performance.
It will provide specific data about all the Internet traffic that goes through to the PC.
- Is compatible with the most important Web browsers: Internet Explorer, Mozilla Firefox, and others.
- Its user interface is known to make it easy to monitor HTTP requests.
- It's extensible using the .NET Framework.
- It supports JScript .NET (an event-based scripting subsystem).
- Has problems when working with an authenticating upstream proxy server.
- It is not able to monitor requests if using a localhost or 127.0.0.1 address (as local addresses typically bypass any proxies).
- Fiddler 2 requires version 2.0 of the .NET Framework. And, Fiddler v2.x loads only .NET CLR v2.0 assemblies.
- Fiddler 1.3 is used for .NET Framework version 1.1 users
For Windows platforms: Windows 2000 / XP / 2003 / Vista with Microsoft .NET Framework v1.1 (4322) or later10 megabytes disk space / 800Mhz Pentium/Athlon processor (Screams at 2ghz)128 megabytes RAM (512 or more highly recommended). Also compatible with Server 2007/8.
What's new: The latest update of Fiddler 220.127.116.11 includes:
- Tweaks for Firefox 5
- Assorted bugfixes