What is the name of the IE plug in that someone can download (I think from Microsoft) that lets a developer (well, anyone who gets the plug-in, actually) to view the message traffic that goes on behind the scenes from the browser to the server? I saw this one in action but I forget its name. And I think, for the FireFox browser, you can simply turn it on somehow without getting a plug in.

It cuts the browser window in half horizonally and the bottom half is also divided vertically and you can see the GET and POST messages as well as the complete header information that is sent to the server from the browser across the internet.