The Library of Congress has a searchable database of free Common Core-aligned lesson plans, primary sources, and other materials available to download and use in your classroom. The database is searchable by state, grade level, and subject. Only English/Language Arts and Literacy for History/Social Studies choices are currently available for Common Core, but more are available depending on your state selection. Users can also search by national organizations, like the National Council for the Social Studies or the National Council of Teachers of English, American Association of School Librarians, National Education Technology Standards, etc.
Thanks for the tweet, @coolcatteacher