BadfingerD is a simple program, written and tested on Linux systems. It should be portable to lots of other platforms as well, though it may need some adjustment.
All it does is listen for a TCP connection (on port 79 by default). When it receives a connection, it ignores input sent by the client. Instead, it executes a shell command and redirects the output back over the TCP connection. ...and that's it. It can be useful for monitoring in-home devices such as file servers, re-flashed access points, or such-like.
That's all there is to it.