Tim and Molly O'Brien – bluegrass, newgrass, gospel; as bro & sis, they have that special "family voice/sound" going for them.

Coope, Boyes & Simpson – British male trio, trad & new songs, wonderful tight harmonies

Graham & Eileen Pratt – mostly trad British songs (half of Regal Slip) – fantastic blend!

The Watersons & the Copper Family – no comment needed, methinks

Sweet Honey in the Rock – six African American women with powerful voices, doing spirituals, hymns, gospel, jazz, blues...

Manhattan Transfer – slick, oh-so-sophisticated harmonies

The Everley Bros – interesting harmonies; great blend

Rockapella (are they still around?)

The Four Bobs – wonderfully tight harmonies, and extremely original arrangements of songs you wouldn't really expect a capella renditions of, like Purple Rain, Psycho Killer and Innagoddadavida(sp?)

Agree with Midchick about Finest Kind and with PoppaGator about the Roches

There's SO many more, of course – but these are the ones who come to mind straight away...