Lovely GNU Screen

A thousand windows open, two monitors because the space runs out otherwise, no Exposé on Windows, and a dozen PuTTYs open to various servers - now just don't lose track (dramatized).
With GNU/Screen I can open several ttys inside one tty. Put in terms for Windows folks: I open an SSH connection in PuTTY, start GNU/Screen and can then jump back and forth between several shells. Naturally you can also run programs in there that you really ought to have open all the time anyway. mutt and top, for instance.
The real killer feature is that I can also detach GNU/Screen and log out of the system. The programs keep running in the background. When I log in again from wherever, I can pull the session back.
You start GNU/Screen with

lemming@deepthought:~ $ screen

After that, something like the following appears

Screen version 4.00.02 (FAU) 5-Dec-03

Copyright (c) 1993-2002 Juergen Weigert, Michael Schroeder
Copyright (c) 1987 Oliver Laumann

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along withthis program (see the file COPYING); if not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA  02111-1307, USA.

Send bugreports, fixes, enhancements, t-shirts, money,
beer & pizza to [email protected]

             [Press Space or Return to end.]

Just hit Enter to continue. Now you have another shell open. This is a window (a “screen”); you open a further screen with a further shell using the key combination CTRL+a followed by c. You can jump back and forth between the two screens with CTRL+a followed by CTRL+a again. AFAIK you can open an infinite number of screens. You get to the first screen with CTRL+a 0, to the second with a 1, and so on.
You close a screen simply by ending the shell with exit, logout or CTRL+d. You then land in the next screen, or, if no window is left, GNU/Screen quits.
There are a multitude of ways to control the windows:

Ctrl+a enters command mode for GNU/Screen.
Ctrl+a c         create a new window
Ctrl+a 0         switch to window number 0
Ctrl+a 9         switch to window number 9
Ctrl+a n         jump one window forward
Ctrl+a h         jump one window back
Ctrl+a a         switch to the last active window
Ctrl+a A         name the window
Ctrl+a '         enter the name of a window to jump to
Ctrl+a "         show a list of all windows
Ctrl+a Ctrl+d    detach GNU/Screen, it keeps running in the background
Ctrl+a h         write a hardcopy to hardcopy.n
Ctrl+a H         log the window contents to screenlog.n
Ctrl+a ?         show a short overview of the keyboard shortcuts

As described above, GNU/Screen can also be detached and reattached later. This is done with CTRL+a d.
A detached GNU/Screen can be brought back with the following command.

lemming@deepthought:~ $ screen -r
There are several suitable screens on:
        19802.pts-3.deepthought (Detached)
        19823.pts-3.deepthought (Detached)
Type "screen [-d] -r [pid.]tty.host" to resume one of them.

However, it may well be that several screens are detached. So you have to be more specific and pass along the PID.

screen -r 19802

GNU/Screen is available on almost every platform, even on Windows with Cygwin. On most Linux derivatives and Mac OS X, GNU/Screen comes preinstalled. On FreeBSD and OpenBSD you can find GNU/Screen in /usr/ports/misc/screen and install it from there.
This program is a real help when you start processes that are supposed to run in the background; don't want to see a dozen terminals on your desktop; or want a log of what went on in the tty.
Small tip: su, fg and bg work flawlessly in GNU/Screen.