LOL, Scoville.

You just reminded me, one place to find commonly-known songs is Sesame Street. Of course, they also use some songs composed just for the show and other recently-written children's songs, but I've heard them do former pop (broadly construed) hits that have made their way into our shared 'folk trove' such as "Catch A Wave" (Beach Boys), "Doggie In The Window," and "Three Little Fishies."

I should add a few more to the well-known "folk" songs list:
When The Saints Go Marching In
Michael, Row The Boat Ashore
This Land Is Your Land
Puff, The Magic Dragon
Roll On, Columbia
Goodnight, Irene
16 Tons
Banana Boat (Day-O)
Hey, Good Lookin'
Wimoweh (The Lion Sleeps Tonight)