Beth's Notes and Beth's Notes Plus (a paid subscription) are terrific resources for elementary general music teachers. With hundreds of pieces of folk repertoire notated and cataloged into detailed indicies, this is a resource to bookmark immediately. Beth is a master of her craft, and this site is far more than a collection of children's songs. Look for her grade specific ideas, her lesson plans, and her ideas for music programs that you can integrate into your yearly curriculum.
I use world music as a way for my students to explore diverse cultures without leaving the classroom. We sing, play, and study musical traditions from around the world, and I find that I'm almost always referring to Smithsonian Folkways recordings for inspiration. Here is a collection of lessons and recordings, categorized by region, from across the globe. Be sure to read the liner notes, available as a .pdf in each recording.
American Roots is a music documentary created by PBS. This teacher guide helps to craft a learning experience for your students based around the film, and offers four lesson plans to accompany the documentary.
The Kodaly Song Web is my go-to resource for finding new tunes to add to my repertoire. The site is designed so you can search by title, topic, tonality, scale, meter, chord progression and more, helping you to find pieces that suit the needs of your lesson. Each piece has a .PDF score attached, so learning new tunes is as easy as can be. This resource is much like a whole shelf of books but is only a click away.
When teaching students folk music, authenticity is always a consideration. In my view, it's vital for students to not only participate in the singing and playing of folk music, but to develop an understanding of the origins of songs in the repertoire. Folkstreams is a resource complete with video clips of music making that represents the diversity of the American music experience. Blues, Old-Time, Cajun, Jazz, and more styles of American music are shown here in their original forms.
Singing work songs is a terrific way for children to become accustomed to learning music in the oral tradition. These repetitive songs lend themselves perfectly to comunicative music making, storytelling, impromptu harmonization, and cultural exploration. I've used work songs in my classes to help students explore musical styles as well, applying instrumental accompaniments to create alternate arrangements of the original work song.