Popular (& free) blog hosting sites


www.edublogs.org
Specifically designed for use in educational settings, with classroom / student friendly features.
Help site: http://theedublogger.com/get-a-blog/getting-started-with-edublogs/

www.blogger.com
Google's blog site, so if you have a Google account (Gmail, YouTube etc) then there is no need to create another account. On the down side, this means that students will have all sorts of usernames, some perhaps not appropriate for class.

www.wordpress.com & www.wordpress.org
Open Source blog hosting. My school's in-house blogs are generated through wordpress.com