It depends a whole helluva lot what you mean when you say folk, but for wildly varying (and mostly inaccurate) values of same you might try:

* BobDylan
* GillianWelch
* Smog
* WoodyGuthrie
* ArloGuthrie
* TheBand
* NeilYoung
* RichardThompson
* LeonardCohen
* SimonAndGarfunkel
* TruckstopHoneymoon / MikeWest (if you can admit that this isn't a novelty act, you're at least halfway to salvation)
* JohnnyCash
* NekoCase
* GregBrown
* GlenPhillips
* TheGratefulDead
* JoshRitter
* OldCrowMedicineShow
* TheViolentFemmes
* TheBeGoodTanyas
* [AniDifranco]
* BrightEyes (perhaps best taken in small doses)
* The ''O Brother, Where Art Thou?'' soundtrack