Web service that makes/displays HTTP request/response (includes POST, TRACE, headers, etc)

For the opposite, a URL which stores what's POSTed to it: http://www.postbin.org/

For example: http://www.postbin.org/zfzfg2 (superhappydevhouse.com's pbwiki's webhooks point there)

If you're using Firefox, I might use Tamper Data before I fed a request through a web service somewhere else; if you're using IE, I might use Fiddler.

I want something like this where you can type in the request body.

That is what I was thinking, but I think telnet really does the trick. This is nice just because it gets around the by-hand formatting on telnet.

If you're on OS X try HTTP Client. Allows you to specify request headers and body.

http://ditchnet.org/httpclient/