Spirited away is a nice application for those of us who like to keep things clean on our Macs. If an application is unused for a set amount of time, Spirited Away will hide the window, keeping your desktop clutter free.
I found this especially useful when I recently set up a Powerbook for my grandma to use. She is just learning computers so this will allow her to just continue to use programs that she wants, and not worry about closing/quiting the applications that she is done with at the time. Too many windows open at one time confuse first time Mac grandmas.