Twang

Doc Watson and David Holt - "Shady Grove"

Doc is perhaps the king of bluegrass guitar.  He has a mighty fine voice, too.

Shady Grove, like many bluegrass standards, evolved from traditional English, Irish and Scottish folk ballads that found a home in the Appalachian mountains and never really left.  Centuries later these same songs can still be heard, albeit in slightly different arrangements and context.

Gillian Welch, Dave Rawlings and The Old Crow Medicine Show - "The Weight"

Bill Monroe and The Blue Grass Boys - "I'm Working On A Building"

Penned by Felice and Boudleaux Bryant, but made famous by The Osborne Brothers, "Rocky Top" is the official state song of Tennessee.

The Seldom Scene - "Wait a minute"

Tony Rice  - "Church Street Blues" by Norman Blake

The deadstrings01 in the comments say it all,

Ever wonder how to make a 6 string guitar sound like a symphony?

Roscoe Holcomb goes "Across the Rocky Mountains" with Pete Seeger in tow.

John Hartford - Gentle on my Mind