EmmyLou, of course. Don Williams and EmmyLou sound great together. The Everyly Brothers, TomPall and the Glasers, and I can't believe the Statlers isn't mentioned yet.
Dar Williams and Lucy Kablonsky sound great together, but anything with Dar sounds good I think
There's a song done for a charity benefit of some type that has Celine Dion, Shania Twain, Gloria Estefan and Carole King doing "You've Got a Friend" that sounds great. Hard to bring all those unique and powerful voices together but there is definitely harmony there.
and of course, My all time favorite 'harmony' group..The Bee Gees