Once in a while it’s useful to find the center of a Photoshop document–either the horizontal center, vertical center, or both. While you could Select All, get the dimensions from the Info palette, then divide them in half, there’s an easier way (one that works more reliably after a long day than your ability to do math).
Activate the rulers with CMD+R (CTRL+R), and drag a guide from one ruler or the other toward the approximate center of the image. As you get close, the guide will automatically snap to the middle of its axis–X for vertical guides, Y for horizontal.