Update: I read somewhere that the Gmail team uses some of these tools.
Came across some of these recently - a few tools that are useful for web development / HTTP and network debugging:
Wireshark (formerly called Ethereal).
It's a packet sniffer computer application - it shows you the content of network packets in the network traffic you want to monitor, for example, incoming and outgoing HTTP traffic (a.k.a. HTTP requests and responses) of your web application.
From the Wireshark site:
"Wireshark is an award-winning network protocol analyzer developed by an international team of networking experts."
I've used Wireshark a bit myself while debugging network traffic of a web application - it's very powerful and can do a lot of things to help debug network traffic at different levels of the network stack (TCP/IP stack), such as HTTP, IP, etc.
From the HTTPWatch site:
"HttpWatch is an HTTP viewer and debugger that integrates with Internet Explorer to provide seamless HTTP and HTTPS monitoring without leaving the browser window."
From the Fiddler site:
"Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.
Fiddler is freeware and can debug traffic from virtually any application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more."