Zellij is described as 'terminal workspace. It has the base functionality of a terminal multiplexer (similar to tmux or screen) but includes many built-in features that would allow users to extend it and create their own personalized environment'. There are eight alternatives to Zellij for Linux, Mac, BSD, Cygwin and Xfce. The best alternative is Terminator, which is both free and Open Source. Other great apps like Zellij are tmux (Free, Open Source), GNU Screen (Free, Open Source), byobu (Free, Open Source) and dtach (Free, Open Source).
Terminator enables the user to run and manage multiple terminal emulators side-by-side in the same window. The main focus is arranging terminals in a grid. (Tabs is the most common default method, which Terminator also supports.
tmux is a terminal multiplexer: it enables a number of terminals (or windows), each running a separate program, to be created, accessed, and controlled from a single screen. tmux may be detached from a screen and continue running in the background, then later reattached.
Screen is a full-screen window manager that multiplexes a physical terminal between several processes, typically interactive shells. Each virtual terminal provides the functions of the DEC VT100 terminal and, in addition, several control functions from the ANSI X3.
Byobu is a GPLv3 open source text-based window manager and terminal multiplexer. It was originally designed to provide elegant enhancements to the otherwise functional, plain, practical GNU Screen , for the Ubuntu server distribution.
mtm is the Micro Terminal Multiplexer, a terminal multiplexer. It has four major features/princples: Simplicity There are three commands (change focus, split, close). There are no modes, no dozens of commands, no crazy feature list.
abduco provides session management i.e. it allows programs to be run independently from their controlling terminal. That is programs can be detached - run in the background - and then later reattached.