So, I've been testing a web callback by using postbin - http://www.postbin.org/ - a pretty great service exactly for this sort of thing.
But I found that I'd really like to be able to replay those requests on a server running locally. A quick look behind the scenes at postbin, and it's open source on GitHub, and runs on google app engine.
I also sent a pull request to Jeff Lindsay, the developer on Postbin so he could incorporate the changes on the main site.
None of that is particularly amazing. But the fact that I did it in less than a half hour is!
Distributed VCS, running apps in the cloud, I hope this is the future of software development.