Handy HTTP(S) response testing sites I've found

[Isildur]

• http://www.fakeresponse.com/
  Example: http://www.fakeresponse.com/api/?sleep=20 will send you a response after 20 seconds
• http://ivanzuzak.info/urlecho/
  Example: http://urlecho.appspot.com/echo?status=301&Location=http%3A%2F%2Fwww.example.org%2F redirects to http://www.example.com/
• https://httpbin.org/
  Example: https://httpbin.org/basic-auth/someuser/somepw challenges using HTTP Basic Auth, accepting only the U/P pair you set in the URL.
• http://requestb.in/
  "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."
• https://www.hurl.it/
  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.