The details:
"Visitors, or ‘WWOOFers’, spend about half a day on a host farm, learn about the organic movement and sustainable agriculture, and receive room and board - with no money exchanged between hosts and WWOOFers. WWOOF is an educational and cultural exchange program. WWOOFing is a way to learn practical farming skills, be part of the organic agriculture movement, and experience the heart of American agrarian culture."

Main site is here, http://www.wwoof.org/, with additional sites for farms around the world!