I still love that Traffic version of John Barleycorn, which was the first version of that song I heard.

Another one that comes to mind is Led Zeppelin's version of Gallows Pole.

And Good Shepherd on a Jefferson Airplane album. (The other version I have of this song is done by Mike Seeger and Peggy Seeger under the title Blood-Stained Banders.)

Or, SPB, are you referring to newly composed songs by rock musicians that could actually work as folk songs? Or perhaps rock songs that already have been interpreted by folk artists, of which there are a number. (Ex: Tim O'Brien doing Norwegian Wood.)