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Twitter
[|Sign Up] (email required, Free) Twitter in the Classroom 100 ways to teach with Twitter

Facebook
Using Facebook in the classroom Great site providing examples and how to of Facebook in the classroom Facebook in Education facebook page

Edmodo
Sign up (no email required, Free) What is Edmodo

Blogs
Wordpress Introduction to Blogging Ways to use blogs in your classroom

Google Docs
What is Google Docs Google docs for educators