The following sites can be specific to particular content areas. Another recommended source is museum. Famous museums like the Smithsonian has free lesson plans for teachers.
Information from the Centers for Disease Control and Prevention on how parents, health professionals, educators, and others can work together as partners to help children grow up to reach their full potential.
This is a website published by a foundation that aims to "celebrates and encourages innovation" in K-12 schools. It focuses on six core learning strategies: "Comprehensive Assessment, Integrated Studies, Project-Based Learning, Social & Emotional Learning, Teacher Development and Technology Integration."
The IRIS Center is a national center dedicated to improving education outcomes for all children, especially those with disabilities birth through age twenty-one, through the use of effective evidence-based practices and interventions.
"Open Educational Resources (OER) are teaching and learning materials that you may freely use and reuse at no cost, and without needing to ask permission. Unlike copyrighted resources, OER have been authored or created by an individual or organization that chooses to retain few, if any, ownership rights."