Serial to Network Proxy (ser2net) 
ser2net provides a way for a user to connect from a network connection
to a serial port. I tried all the other ones I could find and
found them lacking, so I wrote my own. It provides all the serial
port setup, a configuration file to configure the ports, a control login
for modifying port parameters, monitoring ports, and controlling ports.
The ser2net SourceForge
project page contains the source code, among other things, for ser2net.
See that page for more info.
ser2net users
The JUPS project can use
ser2net for remote (or local) control of a serial port UPS from Java.