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.

Run 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 200:

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.