Handy HTTP(S) response testing sites I've found
Example: http://www.fakeresponse.com/api/?sleep=20 will send you a response after 20 seconds
Example: http://urlecho.appspot.com/echo?status=301&Location=http%3A%2F%2Fwww.example.org%2F redirects to http://www.example.com/
Example: https://httpbin.org/basic-auth/someuser/somepw challenges using HTTP Basic Auth, accepting only the U/P pair you set in the URL.
"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."
Makes an HTTP request to an address of your choosing.
Those are just a few examples of things you can do with them; check the home page for each one for many, many other directives. Feel free to provide others you've found.