tmux; The terminal multiplexer. In simple words, it divides a terminal into many sessions, windows and panes. An interesting usage is the session can be detached and re-attached, which is very useful to connect remotely. Because unexpected disconnection does not kill the process running. Also, dividing a terminal window into many panes is convenient when connected through PuTTy to maximize the screen real estate.
Nice summary of commands and keys can be found here.