User-Agent is not exclusive to
http. For example, an
email I just sent to myself came with this
header
User-Agent: Mutt/1.2.5i
and a recent post of mine to
usenet bears the following header
User-Agent: tin/pre-1.4-19990517 ("Psychonaut") (UNIX) (SunOS/5.7 (sun4u))
I'm sure User-Agent can be found many other places as well.