I reccomend, to the Artful Codger, that he check out the Swingle Singers. I, also, think it bears mentioning that almost all Barbershop groups are amateurs, who treasure their musical traditions. Certainly, there are songs that are sung by every barbershopp quartet and chorus. They are called Barber Polecat songs and every barbershoppers knows them by heart. I was doing a strolling job in a large township park when I encountered a Barbershop quartet, all strangers to me. I waited until they started "My Wild Irish Rose" and I, just, started singing with them.
An important fact has been missing from this thread. Barbershoppers are the warmest, most accessable folks as you will find. They sing because they love it. They are proud of the sound they achieve and they take pride in their performances.