Popular (& free) blog hosting sites

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

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