I was hoping to setup a listener in C# using something like this...

TcpListener tcpServer = new TcpListener(IPAddress.Parse(""), 5442);

so that a BYOND game could communicate with external applications.
I don't have any experience in this area, but it looks like you may be able to write your listener to respond to BYOND's world.export topic calls.

This post has some info, and links to another post with the message byte format: forum/?id=701287&display=1#701287

Of course, you could also just write a DLL and use call() to send it more simply.
As DC has mentioned, the preferred avenue would actually be to implement HTTP GET support in your listener, and allow BYOND worlds to Export() to the bound address/port of that. Seems C# offers such support out of the bag:
In response to Stephen001
ahh... missed that one. :) Thanks!