Disper is an on-the-fly display switch utility.
It is intended to be used just before giving a presentation
with a laptop, when all one wants is that the beamer, which
has just been connected, is able to show whatever you prepared.

Disper gives you the option to either clone all detected displays,
or extend the desktop to them. Resolutions are automatically
detected. For cloning, the highest common resolution supported by
all displays is chosen; for extending every display device gets
its highest supported resolution. For special setups requiring
more detailed control, one can still use the standard display
configuration utilites.