To anyone that prefers a CLI over a GUI tmux is a must. When I am developing I usually have one of my monitors exclusively dedicated to my browser and other GUI apps and one monitor solely for CLI and my editor. I have my editor (emacs -nw) in a fullscreen window and various bash sessions in panes. Switching between windows and panes using the keyboard keeps me focused on the task at hand. I never have to remove my hands from the keyboard and grab for a mouse.

Many people have reported that multiple monitors with apps scattered about, producing countless opportunities for distracting you are counter productive. Having one monitor dedicated to the CLI and another to my browser and GUI apps at least allows me to focus on a single monitor for the purpose of developing. I am not a big fan of GUI editors (although your mileage may vary). While there are those that like every feature of an editor shoved in their face - I prefer M-x. ;-)