Quit It for Mac

From Katoemba Software:

Quit It is a system extension that automatically quits an application when its last window is closed by the user. This can be very useful when novice users are working on a computer, who often close a window by clicking on the close box of a document window, thinking the application is quit. This can easily lead to confusion, or memory shortage. Quit It can be fully configured through its control panel, with individual settings for installed applications.