Keith Devens .com

if you have mozilla/firebird
the plugin http://livehttpheaders.mozdev.org/ is quite useful too

" The goal of the project is to adds information about the HTTP headers in two ways:

• First by adding a 'Headers' tab in 'View Page Info' of a web page.
• Second by adding a tool in the 'Tools->Web Development' menu to be able to display http headers in real time (while pages are being downloaded from the Internet.
• Third by letting you edit request headers and replay an URL (beta). Look for the Replay button in the live window

"