Suppose you have a netbook with a small screen (say: 1024×600 resolution) and you want to use some applications that expect higher resolutions.
On Ubuntu for example you can open the terminal (Applications -> Accessories -> Terminal) and enter the following:
$ xrandr --output LVDS1 --panning 1024x768
LVDS1 seems to work for my netbook and is used as an indication for the internal panel in the netbook. Use
xrandr without options to see all possible display outputs. To restore the display, change the panning option to 1024×600 again.