Almost ANY song can be arranged in the Barbershop style (except, I am told, the Jackson Five's "ABC-123", but that's another story ). Here we will include links to external sources of barbershop arrangements (BHS, Mainely Acappella, etc.),

Barbershop Style
Barbershop Theory
Barbershop Arrangers
Learning Tracks
Songbooks
Polecats
Notation Software
Black Influence — by Jim Henry
Lists of Songs