Resize Web Fonts using Mouse n Mozilla Firefox. To increase or decrease the font size of a web site, hold down the Ctrl key and move the wheel up to decrease the size, and down to decrease the font size.
In Internet Explorer (IE 6 and above)The mouse wheel-Ctrl key combinations works in IE as well, albeit with a small difference. Unlike Firefox, moving the mouse wheel up increases font size, and moving the mouse wheel down decreases the font size.
Bonus Tip: Via the Vista Reviewed blog
Resize icons quickly using your mouse wheel in Windows Vista:
The title says it all. Hold down Ctrl and move your mouse wheel up or down to enlarge or shrink your icons in Windows Explorer. When you right click an area of empty space in a Windows Explorer window, you are only given 3 different sizes to choose from.