CommentPress is an open source theme for the WordPress blogging engine that allows readers to comment paragraph by paragraph in the margins of a text. Annotate, gloss, workshop, debate: with CommentPress you can do all of these things on a finer-grained level, turning a document into a conversation. It can be applied to a fixed document (paper/essay/book etc.) or to a running blog.
The Social Media Classroom is a new project started by Howard Rheingold which offers an open-source Drupal-based web service to teachers and students for the purpose of introducing social media into the classroom. The service includes tools like forums, blogs, wikis, chat, social bookmarking, RSS, microblogging, widgets, video conferencing, and more.
In the short time that I have used it, Twitter has grown quickly to play a major part in the way that I interact with fellow colleagues and professionals from around the world.