Screen Leap is a free screen sharing service that I recently learned about through a post by Vicki Davis. To share your screen using Screen Leap just visit the site, click "share your screen," enable the Java applet, and send the sharing code to the person you want to view your screen. The person receiving your invitation code will be able to see your screen when you have Screen Leap activated.
Applications for Education
Screen Leap could be a handy little tool for those times when it is easier to show someone how to do something than it is to tell them how to do it. I'm thinking it could be particularly useful when you're trying to walk someone through setting up an account on a new-to-them web service.
Here are some other screen sharing services you might want to try:
Quick Screen Share