Curriki, the online education community, is building the first website to offer free, open-source instructional materials for K-12. We have thousands of free worksheets, lesson plans, exams, project ideas and activities for English language arts, math, science, social studies, technology integration and other subjects. All of our educational material is contributed by teachers and partners and is free and open source.
If you aren't familiar with the institute, they have loads of free resources for US History teachers at all levels - including primary sources, images, and lesson plans. You can also apply, for free, to be an associated institution which will give you access to even more material!