The screen command. It basically saves your command line session for when you come back. It's sort of a terminal manager, like a window manager. That way, in a single terminal session, you can have multiple virtual terminals going on. It's very cool.
If one uses screen, this shell function (put it into .bashrc) is extremely useful:
function scr { if screen -ls | grep -q Main; then # reattach to Main: screen -xr Main else # name session "Main": screen -S Main fi } upon typing scr, it will check if your main session exists and will attach to it. Otherwise it will create it.