privatetcp is a TCP multiplexer that lets local users create their own TCP servers on convenient port numbers (such as the standard HTTP port 80). The same port number is shared by all users on the computer but each user controls the program that runs when they personally connect to it. For example, each user can have their own web server on port 80, isolated from other users' web servers.
privatetcp runs on Linux, MacOS, FreeBSD, NetBSD, OpenBSD and DragonFly BSD. It requires no configuration apart from the port numbers to use. It is normally started from launchd, systemd, daemontools or another service supervisor.
See the privatetcp(1) manual page for details.