A very fun collaborative site. Students (and others) view images, text or video, and then add their comments through text, telephone, microphone or webcam. Their pictures or avatars show up on both sides of the source screen, and you can click on each one to hear or read their comments. Requires registration, and you can restrict viewers.
A step by step PDF to show you how to use Photostory 3.
Create your own "facebook" type of community. Free, but requires registration and can be restricted to certain users. Easy setup.