Three old airlines that I have fond memories of:

MIDWAY (the first): In the old days, they would offer you a free ticket just for getting someone else to join their frequent flyer club (no cost to join, and the new member didn't have to fly anywhere, just enroll!). My wife and I each got three free tickets out of them (the maximum) before they finally cut the promotion off. Perhaps this unrequired generosity helped contribute to their demise?

PIEDMONT: Had some kind of a special deal for a while where you'd get a certificate for one free roundtrip for every three paid roundtrips you took. The thing is, this promotion was not well publicized, but those of us who were "commuting" on Piedmont every week suddenly started getting a steady flow of free Piedmont tickets in our mailbox, and none of us knew why! Because no one knew about the promotion, it probably added nothing to Piedmont's revenue, which in turn probably spawned the modern-day rule that requires us to register in advance for promotions.

EASTERN: Remember the days before frequent flyer programs were automated? On Eastern, you got a book of frequent flyer certificates, and you'd hand a certificate to the gate agent along with your ticket in order to get your miles credited. On TWA, you got a sheet full of stickers that had your FF account number on them, and you'd paste a sticker on the front of your ticket. Eastern and TWA had a reciprocal FF agreement, so you could earn TWA points on Eastern and vice versa. But the Eastern gate agents would let you use both the sticker AND the certificate, so you'd illegally get credit on both airlines when you flew Eastern. (TWA agents would never let you get away with this.) But, to show that we live in a world where morality rules, the unscrupulous Eastern Airline is long dead, while the morally-upright TWA lives on...sort of.