Posterous Spaces was bought by Twitter earlier this year, but it appears to still be going strong and hasn't changed at all since it was acquired by Twitter. One of the things about Posterous that I have always liked is the ease with which you can create a group blog.
In Posterous Spaces you can allow people to make contributions to your blog by simply sending an email to "yourblog'sname" @ posterous.com. For example, if I created the blog "awesomeblog.posterous.com" I could allow others to contribute to the blog by simply sending an email to "firstname.lastname@example.org." You can choose to moderate or not moderate those contributions. From an administrative standpoint, using the email method of contributing to a group blog is much easier than having to enter permissions for each person you want contributing to your group blog.
Applications for Education
Accepting email contributions to your Posterous Spaces blog means that don't have to spend time walking students through creating log-in credentials for another service. Simply have students send an email to "yourblog'sname" @posterous.com and their posts can appear on the blog. It's the quickest way that I have found to get a classroom full of students contributing to one blog.