As an example I will use a comic from xkcd.  Here’s what it looks like normally:

That’s fine, but the comic itself doesn’t even take up a fourth of the screen.  There are two ways I can make the page bigger.

The first is to go to the zoom button in the bottom right corner of the window.  Clicking on the button rotates between 100%, 125%, and 150%.  Clicking the down-arrow next to the button gives more options. (FYI, that "e" icon next to it is IE7Pro.)

My preferred method is to hold control and then use the scroll-wheel on the mouse to zoom.  Scrolling up (moving the tip of your finger away from you) zooms in, and the opposite zooms out.  (Clicking on the zoom button after using this method brings the webpage back to normal size.)
 

Here’s that same page, zoomed in so the comic fills the window:

Using the ctrl+scroll method, IE supports scaling a page up to 10,000%  Here’s Google.com scaled to that degree:

IE 7′s zooming features was especially helpful before YouTube had a full-screen option, but it’s still helpful for emended flash videos on other websites that don’t have a full-screen option.

Of course, this feature is also handy if you have poor vision, but I like it too, because maxin is easier leaning back in my chair.