Library of Congress has quite a number of great resources for teachers and students. The Student Discovery Sets are favorite amongst my favorite resources that I've featured in the past. That collection of resources was recently updated by the LOC. The recent additions to the Student Discovery Sets include iBooks about the New Deal, scientific discovery, and weather forecasting through years.
Another great resource from the Library of Congress that history teachers should bookmark is The Teaching With Primary Sources Journal. The free online journal features activities for using primary source documents in primary and secondary classrooms. In addition to the teaching activities in the journal each issue includes a feature article about that issue's theme. The theme of the current issue is "historical and geographic thinking." After the feature article teachers will find links to related articles and related resources from the LOC and from around the Internet. The Teaching With Primary Sources Journal is published on a quarterly basis. The archives of the journal are available to browse and read online.