This blog entry is primarily intended for science educators that are interested in starting a blog for their teaching activities. Some of them will not be familiar with blogging and other web 2.0 tools. Not the typical audience of this blog, but I would greatly appreciate feedback, as I'm in the process of writing an article about this topic (in German).
Because a blog could help with teaching students. A blog can serve as a simple noticeboard for lecture topics, homework assignments, etc. It is also a great place to post additional material. But more importantly, students can comment, ask questions or make suggestions. This class by Bora Zivkovic is one example. Oliver Obst pointed out that blogs (and other online tools) can save time (and money) compared to one-by-one interactions in person or via email. The motivation for students is nicely explained in this blog post by Meredith Farkas: blogging or posting comments on blogs is a great way for students to reflect what they have learned and to discuss it with fellow students. Tony Griffith recently talked about why do students find genetics so difficult to learn?. He emphasized that students typically are not taught the research mode, where thinking and problem-solving are more important than learning facts.
Most teaching blogs will be hosted at a university server. The university IT department will probably offer a blogging platform such as MovableType or WordPress. If the university IT department doesn't offer such a service, blogs can also be hosted somewhere else. Edublogs, 21classes and Class Blogmeister not only offer blog hosting, but offer additional features such as a central portal for all blogs, class blogs, access restricted to students, control of student entries.
Many science bloggers are also involved in edublogging, and you find interesting discussions about edublogging both here in Nature Network or in many other science blogs (e.g. here). The Edublogs magazine (e.g. this post on popular edubloggers) is another good starting point.