Unyte is a Skype addon that lets you share your desktop or individual windows with other skype users, so they can see what you are doing. I find it useful to help new players in an online game I play, but have also used it to help people change windows settings etc as i can see what they are doing on their screen and can talk them through it. I can see lots of uses for distance learning.
Skype has lots of useful addons for sharing documents or whiteboards etc. (You will need latest version), I also like the battleships addon which I play with a friend in Greece :)