There is still no standard for the webhooks parameters. Because of this they come in different format (XML, JSON) or with different names ('data', 'payload'). For the project to be really useful, support for specific publishers and subscribers need to be added. Good candidates:
See: http://code.google.com/p/pubsubhubbub/ - A simple, open, server-to-server web-hook-based pubsub (publish/subscribe) protocol as an extension to Atom.
In fact watercoolr is also a hub, but with very simple protocol. Will be interesting to make it acts as a PubSubHubBub compatible hub.
See also: Working with a Custom URL
Two files, subs.rb and pub.rb, with support for messagepub, github and ping.fm added - commits: 99d27d8 , b787ee4 and 64c20fb
Maybe next: http://bot.im/