Watch this video.
What I like is the ease with which user is able to write or draw anything. Also I like the way he is able to use different colors.
I have a Mac and I would like to produce a videos for my clients.
What software do I need?
This could be recorded with Quicktime X's Screen Capture functionality (built-in to OS X 10.6), while drawing on a document in any image editor with a brush tool and color palette (ex. Photoshop). The author appears to be scrolling down the document during the lesson. After recording, you could then edit the film in iMovie if necessary (for example, to crop the color palette and other UI from view in the video.)