Q:

Bought Wacom Intuos4 for teaching through online videos

It came with Photoshop 10 Express, Autocad SketchBook Express and Corel Painter Sketch Pad. They all seem like great tools for creating/modifying art and photos. Is there a better application for drawing using the Wacom Intuos4 for the purpose of teaching, like they do at Khan Academy?

A:

This article tells what Khan Academy uses for their screencast presentations:

Initially, Sal used Screen Video Recorder + MS Paint.
Now, he uses Camtasia Studio + Smooth Draw.

Despite what the article implies, he most likely used a Bamboo or similar tablet for all of the videos.
But I would still recommend using something that is more like a slideshow/presentation format. Sketchbook would work well since:

It supports all the basic drawing/whiteboard-type functions with very natural-looking brushes and pencils, markers, etc.
It supports layers, so you can easily group doodles/sketches and edit/remove them individually.
It has a ruler/compass tool and grid lines that can help you draw charts and graphs.
You can navigate across different documents in a manner similar to a slide deck. 
It's got a very simple/light-weight/intuitive interface.

