In a townhouse n'hood near mine, many of the houses have a pipe coming
out of the wall, and I wonder what it is.

It's a white plastic 4" right angle pipe connector that comes out
about 4 inches** and then turns to point down. Nothing is connected at
that point. It's at the same level as the garden faucet so I guess it
comes out of the ceiling of the basement.

**It only comes out as far as the right-angle piece puts it. There is
no other pipe showing.

Some rows of homes have this, but others don't. The ones built maybe
around 1997 have them, and the ones built 2 or 3 years later don't.
(There are some others that don't that I thought were also built the
same year as the first group, but if the construction year determines
if there is a pipe or not, maybe I'm wrong.)

I've met a couple of the people who live there, but the odds one of
them knows what the pipe is for seem slim to me.