I have been using Magnifier.exe to magnify the screen during presentations.  Before I do, I usually crack a joke about people with weak constitutions and expecting mothers to look away, because the effect can be disorienting. 

I just found out about ZoomIt 2.0.  It magnifies the screen area in a fluid and easy to use manner (just Ctrl + 1), and even allows you to draw on the zoomed in screen.  Perfect for presentations.