"Skype in the classroom is a free directory for teachers who want to use Skype to bring education to life in their classrooms. Join today to share resources, chat with teachers and even pair classes."
"This site was created to serve as a collaborative, objective resource for educators and administrators who are committed to enhancing student achievement. We invite you to share your knowledge, ask questions, and get expert insight into the issues teachers face each day in the classroom."
These are web applications that let you create private (or public, though for educational uses I'm primarily interested in ones that allow you to create "walled gardens") networks to share blog posts, images, videos, websites, and chatboard conversations.