FUS++ is a set of enhancements to Apple's Fast User Switching. Its primary purpose is to replace the bulky, screen-wasting menu item that Apple provides with a more compact one. It can currently be configured to show either a small version of your user picture, your short username, or, like Apple's implementation, the long username.
Other FUS++ features include the ability to run scripts or applications on switching to and from your account. For example, if you are annoyed by the fact that iTunes stops playing and refuses to launch multiple instances when you switch accounts, you could make a simple Applescript to shut it down when you change users.