This article is from the Piano Purchase and Maintenance FAQ, by Isako Hoshino firstname.lastname@example.org with numerous contributions by others.
There are a few electronic tuning aid (ETA) devices on the
market which will assist a piano tuner in doing his or her
job. Typically, an ETA device will produce a series of
pitches to establish the "ideal" tuning of a given piano. A
piano tuner will then match the piano to the device. It is
inevitable to use some aural techniques as well to refine the
tuning. It does not replace the ear, but is an aid to it.
The level of the "idealness" produced by an ETA device is
highly dependent on what kind of device is used. The best
one, the Sanderson Accu-Tuner II (costs you a few thousand
dollars), allows a tuner to measure several parts of the
instrument and it will calculate a reasonable tuning for that
particular instrument. This machine also stores tunings so
they can be used at any time, which is particularly good for
recording studios and concerts as the tunings are consistent
and can be completed quickly.