How can I draw over other apps? I'd like to be able to use my Asus Transformer for a presentation using the HDMI out and I'd like to be able to draw on the screen over videos, pictures and over a powerpoint presentation.  Obviously I can't be drawing and navigating simultaneously with touch but I guess I'm envisioning an "always on top" floating dock or tool bar to toggle between the "paint" function and the normal touch input.  

To be clear I'm not interested in specific "drawing" apps, I'm looking for the ability to draw persistently over any and every open app.  

Is there an app or hack that will accomplish this?
In my old school days, this were trivial because just right behind the projector screen are whiteboards which is sometimes used as a "screen" which can be drawn on with markers. 

As more modern alternative, there are also smartboards that provides a virtual whiteboard to draw on with digital annotations. And one of them can be controlled and viewed from an Android tablet.

If you only need video and slides, then you can run your presentation on the computer running smartboard software and connect to the computer with the Android tablet controller. This doesn't actually allow you to draw over arbitrary apps though.