TCP2Serial
TCP2Serial is a utility application to proxy traffic to/from a serial port over the network. For example, you run TCP2Serial on your mac with a serial port attached and connected to a serial device. Then, you can telnet to the host running TCP2Serial on the port configured in the app and anything sent via the telnet session is sent out the serial port, anything received on the serial port is echoed back across the telnet session.
An example use case: You could use this app on a laptop that has a serial adapter connected to a router/switch that is down. Your network engineering team could them telnet to the computer hosting TCP2Serial to configure the router/switch.
NOTE: More than one telnet client can connect. Text received on the serial port is echoed to all connected telnet clients.
One further note, this app can't do anything without a serial port. Since most Mac's now-a-days don't have built-in serial ports you'll need a USB serial port.