Virtual Serial Port Utilities: For connection between different software suites, ensure RFC 2217 compliance for best compatibility
Windows
com0com (open source): A suite of drivers and tools offering virtual serial ports, serial port redirection, and more.
The related hub4com project offers an RFC 2217 compliant mode
HW VSP 3 (freeware single-port version): A software driver that adds a virtual serial port (e.g. COM5)
to the operating system and redirects the data from this port via a TCP/IP network to another hardware interface, which is specified by its IP address and port number.
ser2net (open source): A program for allowing connections between gensio accepters and gensio connectors
Generally, this would be a network connections to serial ports or IPMI Serial Over Lan (SOL) connections, but there are lots of gensios and lots of options. See gensio(5) for information on gensios.
hubub: A simple TCP data reflector for quickly building distributed applications. It can broadcast data from one client to many (or many to many) with little system overhead.