When I first moved to Wash. D.C. in 1977 and began attending music gatherings (mostly with people in the Folklore Society), the one song that almost everyone knew and would join in on was Utah Phillips' "The Goodnight-Loving Trail". It is still almost universally known in this arena.

I seldom go where people are singing most pop songs, so I can't comment on what the average 'person on the street' might sing.