Google's Ngram Viewer lets you search keywords in millions of books over the span of half a millennium, a useful tool for finding trends over time. For power users, the Ngram Viewer also has advanced options, such as searching for particular keywords as specific parts of speech or combining keywords.

This is Skype's attempt to get classroom's collaborating around the world.  Some people are looking for teachers to help them with specific lessons (i.e. Spanish teacher)  Please take a look through the Skype Lessons link for more details.

The Economics Classroom App provides Introductory Economics students at the high school or university level with access to a variety of high quality resources for learning and studying

This is going to be an amazing tool! Use engaging videos to create customized lessons. You can use, tweak, or completely redo any lesson featured on TED-Ed, or create lessons from scratch based on any video from YouTube. Watch the video to learn how.

Interactive games created by Rice University.  Most of them are Science related but some of them are History.

An extensive, yet simplistic site on different periods of history.  

Glean Comparison Search helps you compare and contrast differing viewpoints of a research topic

Great resource for current events.  

"Tinychat provides dead simple, free to use, video chat rooms that just work!"

I read an article about a month ago on when watching a video or movie in class, teachers were using a chat room to simultaneously discuss the movie. Tiny Chat is only one of many sites where you can create temporary chat rooms for the class and share the link to all of your students. At the end, many of these chat rooms disappear and the info gets cleared. What I liked about Tiny chat is that is also has the ability to show webcams, share Youtube videos, and you can create a whiteboard to share with the group so they can do some brainstorming.