This article is from the Aviation FAQ, by Geoffrey G. Peck email@example.com with numerous contributions by others.
There are two basic types of intercoms -- portable and panel-mount.
If you're an aircraft owner, you should strongly consider a permanently
installed, panel-mounted intercom. There are many brands out there --
investigate carefully. You will probably want to wire the aircraft for
stereo, even if you don't have stereo headsets right away, since the
cost of having an avionics shop wire the intercom can easily exceed the
price of the intercom.
Renters should consider purchasing their own portable intercom.
With a portable intercom, you plug the intercom in to the pilot-side
microphone and headphone jacks, and then plug all the other headsets
(up to 4) into the portable. You will also want to purchase a
push-to-talk switch which will allow you to use your headset's boom
mic with the radios in aircraft which are not equipped with a
Portable units vary from about US$90 to US$300; permanent units seem
to be priced US$100-200 more than the portables.
Good squelch action, overall sound quality, audio entertainment inputs,
ability to mix headset models, sufficient output volume, durability,
and whether the instructor can talk during transmissions from the
left seat (without being heard over the air) are important factors.
By far the most popular portable intercoms from the net's perspective
are the Flightcom IIsx (mono) and Flightcom III (stereo), which can be
bought as two-place or four-place units (there's a small expansion box
for the rear seats). The IIsx typically retails for a little over
US$100. A more deluxe version is the Flightcom III, which offers
stereo audio with a plug-in Walkman or Discman. The IIId offers a
digital clearance recorder, which can "remember" and re-play up to
about 30 seconds of speech at the push of a button. Cute, but not
very useful. Panel-mount versions of the III, and IIId are available
as the 403 (stereo), and 403D (DCR), respectively.
Other brands of intercoms include [listed alphabetically] David Clark,
NAT (panel only), Pilot, PS Engineering, Sigtronics, Softcomm, and
Telex. Regrettably, pilots will often defend their own purchase
choices, whether or not they actually have significant experience with
other intercoms. (The FAQ author does have significant in-flight
experience with all of the brands listed above, and he still
recommends the Flightcom units for overall audio quality, squelch
performance, reliability, feature versatility, and price.)