I do believe someone uploaded The Anthology of American Folk Music in the sendspace thread a while ago, so I would check that out and download it. It's a great compilation of folk music from before and up-until the early 20th century.
Good latter American acoustic folk artists to check out are Woody Guthire, Dafydd Iwan, Pete Seeger, Harry Belafonte, Burl Ives, Odetta, Phil Ochs, Tom Paxton, and Joan Baez.
Later, the blending of folk with other styles of music (mostly rock and roll) became popular. Great examples of this from America, England , Ireland and Canada are Bob Dylan, Simon & Garfunkel, The Byrds, Buffalo Springfield, Crosby Stills & Nash (and Young), Neil Young, Fairport Convention, Gordon Lightfoot, Donovan, Joni Mitchell, and the Pogues.
For modern folk music, I'd check out Bonnie "Prince" Billy and his associated projects, as well as Devendra Banhart.