Most people know how to use the Apple (Cmd) key with “+” and “-” to zoom in and out within a Photoshop document. But if you have a mouse with a scroll wheel, there is a faster way.
Hold down OPTION when you spin your mouse’s scroll wheel, and Photoshop zooms in and out on your image (PC users, hold down ALT).
Tip: To snap to even zoom ratios, such as 25%, 50%, 100%, 200%, etc, keep the Shift pressed as well.
Source: This tip inspired by a recent post on the great CreativeBits.org blog.