How about Woody Guthrie? To me, that's modern because he was writing songs in my lifetime and I consider most of them folk songs because they sound like folk songs. Examples: Pretty Boy Floyd, The Reuben James, Great Grand Coulee Dam -- lots of others.

I think what makes a song a folk song is whether ordinary people are still singing it long after everyone's forgotten who wrote it. By my own definition, most of Woody's songs aren't folk songs YET but I spect they will be. So will a lot of songs by Hank Williams, the Delmore Brothers and lots of early country singers.