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.