http://respondto.it/ is a web hook debugging and stubbing tool. It logs web hook requests, but unlike other tools it also allows the configuration of a default response for JSON and XML requests.
Web hooks have traditionally been (and continue to be) one-way notifications over HTTP. Tools like http://respondto.it/ or http://requestb.in/ exist to make it easier to analyze web hook behavior when implementing a service to process the hook's request. (Think "glorified puts statement.")
Increasingly, web hooks are not only used for push notifications, but can also be used in a workflow scenario: the response to a web hook request can instruct the calling server on what behavior to execute next. A good example of this is Twilio. An entire call flow is implemented through a series of web hooks that respond with XML defining the next set of actions to take.
If the limitations of http://respondto.it/ don't match your needs, you can easily deploy your own version on heroku:
git clone git://github.com/aaronlerch/respond-to-it.git
cd respond-to-it
heroku create --stack cedar
heroku addons:add redistogo:nano
git push heroku master
Then tweak to your heart's content.
Thanks to Andrew Dunkman for some UX help at Codemash