There are so many great Web 2.0 apps, it's almost overwhelming! Here's another one I think might be useful, especially with the explosion of the iTouch devices: edmodo.
"A private social platform for teachers and students to share ideas, files, events and assignments."
What I like about edmodo is that is is a great solution for teachers wanting to create an online community with a bit more security. And, students don't need an email address to sign up; the teacher creates the group, generates a login code and students can quickly register themselves. The teacher can then change the code to prevent "unauthorized" kids from joining. The other bonus is that you can access edmodo via an iTouch.