Sketchcast has just launched, and in my view it is the most intuitive web tool for teachers ever created. It works just like a markerboard. You write or draw, and talk. It is all recorded and can be played back at any time. The service is still pretty basic, and I suspect it will take off so fast that their server will get overwhelmed sometime, but I still urge you to give it a go. Here is my first attempt: it’s a bit wobbly, but it does seem to get the point across reasonably well.