Comment on MacLoc

@generousdesignr : This action by default produces a black screen in Snow Leopard, no the login screen.

But, your observation that this behavior is included in the operating system in both Leopard and Snow Leopard is correct.

System Preferences > Accounts > Login Options

You can check "Show fast user switching menu as:" and then select between Name, Short Name, and Icon (depending on how much of the dock you mind it taking up).

If checked, there will be a drop down menu option that says "Login Window..." that will do exactly what this app does (suspend the user's session).