I love disper!

...and, if you use a laptop with nVidia graphics and want to extend/clone your screen output to an external monitor or projector, you will, too.
Disper is a python-based command line app and can be downloaded from here. I don't think there are any prerequisites except the nVidia packages and basic Python, and potentially xrandr. Install with make /sudo make install.
The beauty of disper is that it automatically and quite intelligently fits your screen resolution and panning to the detected displays.

Extend to an external monitor:

disper -e

Clone, e.g. to a projector:

disper -c

Revert to single-screen:

disper -s

For more options, see disper -h.
Disper can get befuddled by removing the monitor cable, I think; in that case, restarting X helps. Set khotkeys shortcuts for quick switches.
Notes:
I got the tip from this thread - btw, creating a dual-monitor xorg.conf as proposed there is not necessary.
Usually, Plasma should set up a full-blown activity on the external monitor, including an independent background image, widgets, proper window maximisation and so on (cf. my screen shot). Sometimes this didn't work for me; I could move windows to the external screen, but had no background, widgets or maximisable windows. Using disper -S before disper -e helps for some reason. Sometimes Plasma chokes on the disper operations, restarting Plasma (without reverting to single-screen with disper) helped in my case.