B*Witched also use quite a lot of traditional influence in their music. Just listen to things like "C'est La Vie" or "In Fields Where We Lay" from the Awake And Breathe album and you'll see what I mean. They also got the help of Ladysmith Black Mambazo for "I Shall Be There" to stick a bit of African traditional music in there.

Does Eva Cassidy count as folk?