On terminals and tablets where multiple users need to use/share the same machine in a busy environment it could be useful if the system allowed you to switch between user accounts similarly fast as you can do between sites. It is a common situation that there are far fewer terminals than users in these types of environments.
This feature could work with a simple dropdown next to your username. Every time you log into the system with another username the system would store that in a cookie and display all the users in the list who have used the system from that device.
You would then only be asked to input your password and you would automatically logged to the other account.