Robin Good reviews Citrix’s GoToMeeting. It seems to be a pretty impressive tool, though not Free, so perhaps not ideal for students to experiment with. He also links to a post he made a few days ago, which has an overview of what he feels are the differences between “Screen sharing” and “application sharing” – and all the associated features. Some are totally free, some definitely low cost (e.g. $0.10/participant/meeting), others are $30+ a month.