requestb.in highly recommended

Wrote in python with very simple UI and functionality, but provides a quick way to test callback URL without setting up a web server. From its website description: RequestBin lets you create a URL that will collect requests made to it, then let you inspect them in a human-friendly way. Use RequestBin to see what your HTTP client is sending or to look at webhook requests.

When we inspect outbound request, we usually use HttpAnalyzer, fiddler, and other network utility tools. But when we try to inspect inbound request, we usually need setup server, capture log, use tcp dump or other network utility tools. Now we have requestb.in hosted service, it is very handy to inspect inbound http requests with headers and content.