The links below are categorized to help you find Web 2.0 tools that might be of use to the K-12 classroom teacher. (Disclaimer: It is highly recommended that the individual teacher carefully preview any sites before sharing materials in the classroom with students!)
Check out the comments for several tools and services that aren't included in the list.
It's Part 1 of "Back to School with the Class of Web 2.0″, with the other parts detailing online applications, and the real-time use of these services in classrooms and learning institutions.