xrandr is a simple command line interface to X's RandR extension, allowing you to manipulate the root window and map it onto different devices. I use it to switch between my laptop's builtin LCD screen and external monitors when I'm giving presentations, or when I'm at home and just want to take advantage of a larger display.
You can get a list of available devices with
$ xrandr
Screen 0: minimum 320 x 200, current 800 x 480, maximum 4096 x 4096
LVDS1 connected 800x480+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
800x480 60.6*+
640x480 85.0 72.8 75.0 59.9
720x400 85.0
640x400 85.1
640x350 85.1
VGA1 disconnected (normal left inverted right x axis y axis)
TV1 disconnected (normal left inverted right x axis y axis)
Once I've plugged an external monitor into the VGA port on my laptop, I get
$ xrandr
Screen 0: minimum 320 x 200, current 800 x 480, maximum 4096 x 4096
LVDS1 connected 800x480+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
800x480 60.6*+
640x480 85.0 72.8 75.0 59.9
720x400 85.0
640x400 85.1
640x350 85.1
VGA1 connected (normal left inverted right x axis y axis)
1280x1024 60.0 + 75.0
1152x864 75.0
1024x768 85.0 75.1 60.0
832x624 74.6
800x600 85.1 75.0 60.3
640x480 85.0 75.0 60.0
720x400 70.1
TV1 disconnected (normal left inverted right x axis y axis)
Shift output to the external monitor at its full resolution with
$ xrandr --fb 1280x1024 --output VGA1 --auto
$ xrandr --output LVDS1 --off
When you're done, shift it back with
$ xrandr --fb 800x480 --output LVDS1 --auto
$ xrandr --output VGA1 --off
If you want to split your screen across both monitors, use something like
$ xrandr --output VGA1 --auto --right-of LVDS
For more information, check out the excellent tutorial at ThinkWiki.