I have been looking for a way of adding highlighting to web pages for a while now. In PowerPoint (or any office app) the inking is built in to Tablet XP.
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After much searching I found M2's Screen Ink. I have been using it for about 3 month's now.
It is a great little app but it has not been developed since 2003. One of the best things I have found about it is when it is used in 'Live' mode you can add ink on top of video's or animations. This makes using web pages in class, with the wireless projector, much easier. I can add emphasis where I want as I talk.