Yugma: free web conferencing, forever, for Windows and Mac
Yugma's strapline says it all: free web conferencing, forever, anytime, anywhere, Windows and Mac. It's a beautiful web-based tool for video conferencing, the kind of thing that doesn't need an install from your school IT Technician. It took about a minute to open up, register and get going. Brilliantly clear, with the option to share your desktop (so you can draw, write, illustrate) and a nice set of annotation tools to 'point' out what you're talking about.
It's another example of the web platform becoming the free web platform. If you want it, you really can get it for free. Now, all I need is someone in a school with whom I can try it out.