Update! Click here (Zoom, Pan, and Navigate Documents in Photoshop CC) to see all zoom, pan, and navigation tips for Photoshop CC in one post.
If you zoom into an image above 800%, a Pixel Grid is displayed on top of the image. This can be especially helpful when trying to align shapes such as rectangles so that they begin and end on a full pixel (to avoid anti-aliased edges). However, to toggle this off, you can uncheck Pixel View under View > Show Pixel Grid).