yugma

www.yugma.com

Free (up to 10 users) desktop sharing web application that works across platforms.

Students or teachers can use this application to collaborate in real time- watching a doucment being edited, then sharing the document.

Presentations can also be shared on the desktop, and presenter's control can be passed around among participants.  This feature would be ideal for practicing a group presentation one more time the night before you're in front of the class!