Olá, collo o fío por aquí por iso de non chegar á 'profundidade máxima 
dos nosos lectores de correo'. 
Visto que non se chega a consenso, vou facer un repaso con máis profundidade 
do asunto replantexar a miña postura, que espero que poida ser de consenso. 

En esencia, as propostas que se barallaron:
1. Non hai diferenzas conceptuais entre os temos logo a tradución debe ser 
igual
2. Diferentes combinacións de 'complemento', 'extensión' e 'engadido' para 
'add-on', 'plug-in' e 'extension'. (creo recordar).

Persoalmente, creo que por motivos prácticos hai que definir termos separados 
pero tamén por motivos conceptuais, que son só matices pero que poden chegar 
a ser necesario usalos. 
Primeiro, exactamente como se define cada termo: plug-in, add on & extension 
(Nota: procurei tanto 'add on' coma 'add-on' e 'plugin' e 'plug in', por se 
houber diferenzas)


Add-on

Collins Cobuild English Dictionary
add on
If one thing is added on to another, it is attached to the other thing, or is 
made a part of it.
Holidaymakers can also add on a week in Majorca before or after the cruise...
To the rear is a large dining room.added on early this century.
PHRASAL VERB: usu passive, V P n, V-ed P
If you add on an extra amount or item to a list or total, you include it.
Many loan application forms automatically add on insurance.
PHRASAL VERB: V P n (not pron), also V n P

Collins Cobuild English Dictionary
add-on
An add-on is an extra piece of equipment, especially computer equipment, that 
can be added to a larger one which you already own in order to improve its 
performance or its usefulness.
Speakers are sold as add-ons for personal stereos...
N-COUNT: oft N n

Oxford Advanced Learner's Dictionary
add-on
noun
a thing that is added to sth else:
The company offers scuba-diving as an add-on to the basic holiday price. * 
add-on software (= added to a computer)

WordNet
add-on
n 1: a component that is added to something to improve it; "the
addition of a bathroom was a major improvement"; "the 
addition of cinammon improved the flavor" [syn: {addition},
{improver}]
2: a supplementary component that improves capability [syn: {accessory},
{appurtenance}, {supplement}]

Plugin

Collins Cobuild English Dictionary
plug in
(plug in or plug into)
If you plug a piece of electrical equipment into an electricity supply or if 
you plug it in, you push its plug into an electric socket so that it can work.

PHRASAL VERB: V P n (not pron), V n P, V n P n
If you plug one piece of electrical equipment into another or if you plug it 
in, you make it work by connecting the two.
PHRASAL VERB: V n P n, V P n (not pron)
If one piece of electrical equipment plugs in or plugs into another piece of 
electrical equipment, it works by being connected by an electrical cord or 
lead to an electricity supply or to the other piece of equipment.
PHRASAL VERB: V P n, V P
If you plug something into a hole, you push it into the hole.
Her instructor plugged live bullets into the gun's chamber.
PHRASAL VERB: V n P n


Free On-Line Dictionary of Computing
plugin

Responderlle a