VGA dummy plugs were made popular by
folders. They are electronic devices
used to fake the presence of a monitor attached to a VGA or DVI output port so
that the graphics driver can be forced to initialize the corresponding GPU in
order to be able to run GPGPU programs on it —Folding@Home being one of
the most popular. A dummy plug works by simulating a resistive load of 50-150
Ω on the analog red, green and blue VGA signals. Many people
build pretty complex dummy plugs,
load on all 3 signals,
soldering resistors, and
in VGA connectors.
I prefer a much simpler technique: simulating load on one signal,
without soldering, or any extra hardware. I simply insert the resistor
—75 Ω is the ideal spec— in the DVI output port to make contact
between C1-C5 (analog red - analog ground), or C2-C5 (green - ground),
or C3-C5 (blue - ground).
[Update 2011-07-08: added schematic].
The photo below demonstrates a resistor in C2-C5. I have tested
the method successfully with ATI HD 4850 and ATI HD 4850 X2 cards.
It made sense for drivers to not enable a GPU when no monitor was attached to
it. But since the rise of GPGPU applications, graphics vendors AMD and NVIDIA
realized that this behavior was inconvenient. At least with the most recent
Linux drivers, dummy plugs are not necessary anymore.
[Update 2011-02-22: However Windows still requires dummy plugs,
as of February 2011.]