Via the IE team blog, it's Fiddler! This tool looks great.
When I worked on the Sharepoint Portal Server client components, I often spent a lot of time with the Windows Newtork Monitor tool, sniffing HTTP packets to see what requests Office XP or other clients were sending, and what responses the server was providing. I also used a little tool that a tester had built to issue custom HTTP (mostly DAV) requests.
Fiddler looks like a much more refined tool for this kind of work, though, and I'm looking forward to trying it out. The fact that it's built on the .NET Framework is a nice bonus.