Blogger
A free, easy-to-use blogging tool. Blogs can be integrated into your class to promote student reflection, exchange of ideas and/or demonstration of key concepts. Sign up for the "Edublogs" iTech Lab workshop to learn more!

EduBlogger is a fully featured blogging system that is free (it looks like for a one semester trial). It has many of the same features as any existing blog system, but it is structured specifically for classrooms.

WordPress
Another very popular blogging site!