Lock your Mac Desktop with the Locker Widget
Locker is a very simple Dashboard widget that does something Mac OS X should have included by default, it instantly locks your Mac desktop.
All you have to do is open your Dashboard and double click the Locker icon and you’ll end up at a login window where you need to enter a login and password to regain desktop access.
The uses for this are many, but I am still surprised Apple does not provide a simple way to lock your Mac desktop with Fast User Logout outside of either a screensaver or fast user logout with Shift-Command-Option-Q.
Anyway, check out Locker, it’s no frills and gets the job done.