Free: The mission of Free is to make the learning resources of the US Government easier to find. There are over 1500 lessons, objects, and tools for teaching from the federal government located here.
AMSER: Materials in the Applied Math and Science Educational Repository are free for use and adaptation. Most resources are at the high school and community college levels.
Edsitement!: Learning objects and lessons from the National Endowment for the Humanities. It offers a large collection of peer evaluated websites, mostly focused on K12.