Slideshare is a site where you can host your presentations and share them with others. Presentations can be linked to at the site itself or else embedded in a web page. You can also synchronise an MP3 audio file (podcast) with the slideset to create a slidecast - a more powerful way of distributing presentations/tutorials.
Keep in touch with colleagues and friends in less than 140 characters. Good for keeping track of current trends, publicizing events and polling (in and out of the classroom).
Edublogs is "the largest education community on the Internet" where you can sign up for a free WordPress-powered blog. The site provides lots of useful information, e.g. how to use your blog with your students and how start a conversation using your blog. An ideal site for educators who want to blog with support.