As the name implies, this script simply centers the active layer within the document, both vertically and horizontally.
Download
To download this script, right-click (Ctrl-click on the Mac) on the appropriate version below, and choose "Save Target As".
Center Layer
Version: 1.1.0 (July 9, 2009)
Download for:
Photoshop CS or
Photoshop CS2+
Note: The only significant difference between the CS and CS2+ versions of this script is the extension. Photoshop CS scripts have a ".js" extension, while scripts for Photoshop CS2 and higher use ".jsx".
Installation
Download the script that corresponds to your version of Photoshop, and copy it into your "Adobe Photoshop CS#/Presets/Scripts" folder.
After restarting Photoshop, the script should appear in the File » Scripts menu.
Tip: Did you know that you can use the Edit » Keyboard Shortcuts command (Ctrl/Cmd+Alt/Opt+Shift+K) to assign keyboard shortcuts to your scripts? You can also record scripts within actions and then assign function keys to the actions.