Quite often, I have two presentations open, one which is the one I need to build, and then others from which I copy slides. Also reviewing documents, I often have my mind mapping program open to take notes (Mindjet Mindmanager is great), or have my notes open when writing a document. Or two drawings to compare dimensions and see if things will fit. Or in the case of a CAD layout, the schematic and layout in different screens. it really is wonderful, and you can always minimise everything on the second screen to have the tasteful desktop background showing.