HttpWatch and HttpFox are plug-ins for Internet Explorer and Firefox that provide detailed analysis of the HTTP requests and responses.  HttpWatch 6.0 works with both Internet Explorer and Firefox while HttpFox is strictly for Firefox.  The functionality provided in both tools is very similar and extremely valuable when understanding application performance.  Lori has previously posted a review of HttpFox so I won't rehash what has already been said. 

The one key feature that HttpFox is currently missing at the moment is the ability to export or save the results. This has been accepted by the development team as an enhancement so keep your fingers crossed that this is available soon.  In my day to day work I am frequently asked for help in troubleshooting WebAccelerator.  It often times isn't possible to get direct access to the application this is where the ability to save and export the HttpWatch traces become invaluable.

Seeing a trace of an application taken directly and then through WebAccelerator I can quickly see the application mime-types, the cache settings, request and response headers, and the content of the page.  Being able to compare a before and after snapshot of an application can reveal what is wrong.  The type column and headers reveal if the content is matching to an incorrect node in a policy and receiving the wrong acceleration rules.  The Time Chart reveals whether a timeout is occurring due to server side generation delays.  And the Cache tab can reveal whether the timestamps are incorrect somewhere along the chain.  Overall the existence of tools like HttpWatch and HttpFox make the troubleshooting process much easier.