Not only can you listen for incoming HTTP Requests with netcat, you can also be a good citizen and respond to them. The client making a HTTP request will typically wait for the server to respond; although, it should eventually time out.
nc -kl $PORT and when you see an incoming HTTP connection, paste or type
in the properly formatted HTTP response you want to provide when you see
incoming HTTP connections. This is useful for debugging clients that post
updates over a callback URI.
Many clients' APIs are cool with an empty response as long as it has status code
HTTP/1.1 200 OK Content-Length: 0
(note that for a proper HTTP response, you need two line feeds)
There is probably a simple way of piping netcat the response periodically,
without manually pasting it in; however, On OS X,
while true; do echo -e $HTTP_RESPONSE | nc -l $PORT; done
would often result in an unexpected EOF error in my client from time to time.