The Google Cultural Institute offers exhibitions featuring images, documents, and artifacts from significant cultural events of the last one hundred years. 
History Animated offers animated online tours of notable battles and strategies and the outcomes of each battle in the American Revolution, the War of 1812, the American Civil War, and WWII.  History Animated's maps show troop movements throughout the wars.
The Smithsonian's History Explorer is packed with lesson plans, interactive media, and reference pages for teachers and students.
Crash course is an educational Youtube Channel that explains things clearly in animated videos.
C-Span Classroom offers free, primary source materials for teachers to use.
iCivics offers educational game for students. The games introduce students to the roles of citizens and government in the United States. Students who register for a free iCivics account can pause the games and come back to them at a later time.
History.com provides videos, photos, audio, news, date related events and more.  
Timelines.tv is a resource for history teachers to bookmark and share with their students. On Timelines.tv you can find timelines of important eras in U.S. and European history. Each timeline includes short videos about people and events in the era. 
TimeMaps has been described as a mash-up of encyclopedia, timeline, and map elements.  TimeMaps' world map is designed as an overview of the development of the world's societies.
A link to a .pdf with many resources for world history teachers.
This is a collection of educator communities that discuss Social Studies topics.
Hip Hughes History has over 300 videos to help students learn about a wide variety of social studies concepts and events.  
Tom Richey has many videos, powerpoint lessons and lesson resources for a number of Social Studies courses.