Hold down the Control (Ctrl) key, and scroll your mousewheel “up”. Your screen will zoom in on the area around your mouse. Release the control key, and you can pan around the screen by moving the mouse. You can zoom in so close that a single character takes up the whole screen.
To go back to normal, hold control again and scroll out (down).
It’s very cool - I’ve been using it show things to people across the room that they otherwise would have to come close up to see (i.e. Dashboard widgets).
Another neat thing is that Control-command-option-8 switches the display to inverse black & white. The control panel can also switch it to inverse color, and it has options for zooming, for those of us without mouse wheels.
On a laptop, the scroll wheel isn’t necessary, since using two fingers instead of one on the touchpad is the same as using a scroll wheel. That inverse colour business blew my MIND, though.
I’ve found it’s fantastic for keeping eyestrain away when I’m working with a lot of text. I toggle the color scheme every hour or so to keep my eyes adjusting, and the headaches are further apart and far less severe.