Tmux

Install tmux.

Use macPorts for the install process. It can be done like this at the time of writing they page.
sudo port install tmux
type in your password, and you should get something similar to this:
Password:
--->  Computing dependencies for tmux
--->  Dependencies to be installed: libevent
--->  Fetching archive for libevent
--->  Attempting to fetch libevent-2.0.22_1.darwin_15.x86_64.tbz2 from http://lil.fr.packages.macports.org/libevent
--->  Attempting to fetch libevent-2.0.22_1.darwin_15.x86_64.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/libevent
--->  Attempting to fetch libevent-2.0.22_1.darwin_15.x86_64.tbz2 from http://nue.de.packages.macports.org/macports/packages/libevent
--->  Fetching distfiles for libevent
--->  Attempting to fetch libevent-2.0.22-stable.tar.gz from http://skylink.dl.sourceforge.net/levent/libevent/libevent-2.0
--->  Verifying checksums for libevent                                               
--->  Extracting libevent
--->  Applying patches to libevent
--->  Configuring libevent
--->  Building libevent
--->  Staging libevent into destroot
--->  Installing libevent @2.0.22_1
--->  Activating libevent @2.0.22_1
--->  Cleaning libevent
--->  Fetching archive for tmux
--->  Attempting to fetch tmux-2.1_0.darwin_15.x86_64.tbz2 from http://lil.fr.packages.macports.org/tmux
--->  Attempting to fetch tmux-2.1_0.darwin_15.x86_64.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/tmux
--->  Attempting to fetch tmux-2.1_0.darwin_15.x86_64.tbz2 from http://nue.de.packages.macports.org/macports/packages/tmux
--->  Fetching distfiles for tmux
--->  Attempting to fetch tmux-2.1.tar.gz from http://lil.fr.distfiles.macports.org/tmux
--->  Verifying checksums for tmux
--->  Extracting tmux
--->  Configuring tmux
--->  Building tmux
--->  Staging tmux into destroot
--->  Installing tmux @2.1_0
--->  Activating tmux @2.1_0

If you want integration with system pasteboard consider installing port tmux-pasteboard as well

--->  Cleaning tmux
--->  Updating database of binaries
--->  Scanning binaries for linking errors               
--->  No broken files found.