Webhook Tester : Webhook Tester allows you to easily test webhooks and other types of HTTP requests.
Requestdiff : Differentiate HTTP headers and content body of two GET URLs.
patchbay : patchbay.pub is a free web service you can use to implement things like static site hosting, file sharing, cross-platform notifications, webhooks hand...
HTTP Archive : The HTTP Archive tracks how the Web is built.
Minimize round-trip times : An article on the best practices to apply in order to minimize HTTP round-trip times
RequestBin gives you a URL that will collect requests made to it and let you inspect them in a human-friendly way.
Use RequestBin to see what your HTTP client is sending or to inspect and debug webhook requests.