WebhookInbox is a web service and website that receives HTTP requests from external sources and stores the data of those requests for later retrieval. Each inbox is represented by a generated URL, which can be passed to other applications for use. If you're developing a service that makes HTTP callbacks, WebhookInbox can be used as a convenient way to examine what you're sending. Incoming request data is displayed on the WebhookInbox website in realtime as it happens.
There is also a REST API available, meaning WebhookInbox can be used programmatically, for example by a browser app wishing to receive HTTP callbacks.
The URLs generated by WebhookInbox can even be used with PubSubHubbub. You can create an inbox and subscribe its target URL to a PubSubHubbub service, and feed updates will be delivered to the inbox.