Kim Gräsman let me know that he and Jonas Blunck have updated Developer Playground.  You can get Developer Playground 1.1 at either Kim's or Jonas' site (identical files).

Then a few days later, Jonas released ieHTTPHeaders 1.5, with support for color customization and sniffing the request body.  ieHTTPHeaders has helped me debug many an evil cookie problem, and is a must-have for anyone aching to see what's really going on behind the scenes.  There are lots of HTTP Sniffers out there, some more complex, but Jonas' is simple, elegant, functional, and most of all, it seemlessly integrates with IE as an Explorer Bar.  Cool.