This is a "isn't this really old piece of software pretty darn cool," kind of a post. GNU Screen is a terminal multiplexer, that dates probably from the eighties or there abouts, and it provides a sort of text-based windowing environment inside of a command line. Sort of.
Before I started using it, I read statements like that and had no clue what Screen really did. I think a brief (and basic) overview of how screen works might be worth something. Basically, you start a screen instance in a terminal window, and you're brought to a blank terminal window. The commands are, by default all bound to Control-A (c-a), and subsets of that. So you have a terminal open that you can run console applications, or other shell commands. You can also hit c-a c-c to open a second "window" in the same terminal emulator, and c-a c-a swaps between the present and most recent window, while c-a c-" presents a list of open windows. All of this runs within one instance of a terminal window, so you don't have to resort to tabs, awkward key bindings, any of it. Everything is there.
There are a lot of additional features, most of which I don't use--I must admit, but the truth is the basic idea of taking a terminal window--which is by nature a single-purpose and single-task--and make it possible to perform many different tasks inside of one window isn't a great technological or user feature in 2008, but there are a few nifty things that make it incredibly useful.
First, screen instances run as daemons (actually, I'm not sure this is the correct term, but nevertheless) so that you can detach a screen instance from the terminal it's running in, and reattach it later. We can imagine this to be useful in a number of instances. First, if you're working over SSH, you can not only have multiple tasks running over a single connection (multiplexed) but if the connection drops, or you need to move computers... your state is saved. Similarly, if you switch terminal emulators (xterm and urxvt, or gnome-terminal) you can save where you are. Screen makes it possible to log-in and of your system without loosing where you were. Commands that are useful in these workflows are: screen -ls to list existing screen instances, screen -r ## to reattach a detached screen (if there's more than one detached screen then specifying a unique PID number or fragment will let you pick between multiple "screens,"). You can also specify a -D flag to detach the screen, and -RR to "force detach/reattach," though I often run screen -DRR just for good measure.
Basically the upshot of this functionality is that all of my terminal applications and work can be disassociated from a specific session or terminal emulator. While this might be my own particular oddity, there's something that I rather enjoy about the independence of being able to separate the processes, from environments either physical (what hardware I use, given SSH) or from specific environments on the hardware that's in front of me (which has certain stability and security features that are appealing.
In the past few months I've taken to running several network connected console apps (the mcabber jabber client, and irssi, the IRC client) in screen instances so that if I needed to restart X for some reason, I could without popping on and offline. More recently I've been using it to cut down on the number of terminal applications I have running at any given time, as the terminal emulators are rather bulky programs in comparison to screen and the shell interpreter.
That old technology, it's worth something. I know there are other screen folks out there, there must be. What--to you--is the "killer feature that I left out?"