Get Info on a Mac Without Logging in

To find out a Mac's serial number, don't bother hunting for the tag on the machine's enclosure, as it's right there in the "About This Mac" dialog box. But what about looking up this information without being logged in?

The standard way to conveniently look up a Mac's serial number is to select About This Mac from the Apple menu and click on the version number below the "Mac OS X" tag: with every click, the displayed information will cycle between the OS version, its build number, and the Mac's serial number.

In the corresponding spot in the login dialog box, the machine's Computer Name as defined in the Sharing preferences is displayed. Click on this machine name, and the text will, again, cycle through the OS version, the OS build number, and the Mac's serial number. Keep on clicking, though, and it will also reveal the current IP address, network accounts, and system clock's date, time, and time zone, before cycling back to the machine name.

The authentication dialog that is shown when the screensaver is locked does not provide access to this info, but via the Switch User… button, you can bring up the login dialog box from a locked screen, as well.

Being able to determine the machine's IP address this way may be all you need for attempting a remote login from another Mac (apart from that other Mac, of course) in case the regular login fails. As for the other tidbits of machine information like the date and time being shown, I don't really know what they could be useful for. Do you?