If you have your login window set to show the Name and Password fields (as opposed to the "List of users" or auto-login), you will see a window like the one shown above when your Mac first boots, when you logout, or when you change to the login window if you have Fast User Switching enabled.
OS X has supported several "special" login names for a long time. Each is entered into the "Name" field with no password. They are:
You can probably guess what the first three do. The fourth one ">console" will drop you to a shell login prompt, where you can login with your username and password if you want quick access to a shell instead of the full user interface. The restart/shutdown/sleep buttons will only work if you have not turned off to option to show them, but ">console" works regardless of that setting.
The upshot of these settings: if you somehow found yourself using a version of OS X which lacked a "Restart" button on the login window, you could login as ">restart" in order to reboot your Mac.
You can also easily add your contact information to the login screen (as shown above).