The Copper family is worth a mention as they both harmonise and also sing "Straight".
If you're interested, there's a wealth of material on topic record's "Voice of the people" series - though some of the songs come across a little impaired by the age of the singers. Still, it's the songs isn't it? :-)

There's a compilation called "Voices in Harmony" which you MUST GET as it has tracks by many of the people mentioned on this thread.

Cheers - Keep on singin'.

CamoJohn