tmux: Open Source terminal multiplexer

From the tmux website:

tmux lets you switch easily between several programs in one terminal, detach them (they keep running in the background) and reattach them to a
different terminal.

tmux is intended to be a simple, modern, BSD-licensed alternative to programs such as GNU screen.

This release runs on OpenBSD, FreeBSD, NetBSD, Linux and OS X and may still
run on Solaris and AIX (although they haven’t been tested in a while).