Where can I download a simple Windows GUI app that can consume an HTTP POST, display its contents, and return 200?
This is really best done by writing a script and loading it into your favourite web server (IIS, Apache, whatever), and then pushing the POST to this file.
Credit where credit's due, ASP code blatantly lifted from here
If you need a more of a "catch all" response (i.e. you want the web server to run this script regardless of whatever URL you're POSTing to) this can be done with a rewrite rule.