Deleted an earlier comment because some thoughtful people here made me realize the error in my definition of folk music. The reality is that because we are a pluralistic society, it’s not possible to point to any songs that we all know. There a multiple folk traditions here, some of which are intertwined with spiritual beliefs, class, race, and even gender. Sometimes with all of these! A lot of songs here are know by a lot of Americans, but there will be regional/racial/religious differences in the level of familiarity we have, too. Personally I think that’s pretty cool. :)

There’s some great responses here. I recommend you look them up on Wikipedia, though, because a lot of people are conflating folk songs and church songs. Non-Christians will likely not be familiar with I’ll Fly Away or Michael, Row Your Boat Ashore, for example because those are both Christian songs. There are also a lot of protest songs represented here (Sixteen Tons, for example—one of my faves).

Some that are unequivocally folk songs that most of us know are On top of old smokey, She’ll be coming ‘round the mountain, Twinkle twinkle little star (but this is usually sung as a lullaby), row row row your boat, pop goes the weasel. These are all kids’ songs.

This land is your land wasn’t technically a folk song because we know who wrote it and it’s only about 80 years old. But it’s definitely becoming one over time. I think Woody Guthrie would want people to think of his song as public property!