Reading a Mac mailing list I found a subscriber that asked how to prevent access to the Mail.app application (and so to it’s data) on an iMac shared with four other people on a front desk in an art gallery.
Another user suggested him the usage of MacAppBlocker, from KnewSense Software:
With Mac App Blocker, you can password-protect EACH application on your Mac. Keep your apps and your Mac safe. Set a timeout value to automatically exit the protected application so even when you leave your computer unattended, you’re still protected.
Personally I’ve never been in a situation needing a solution like this one, but I’ve thought about writing down a note on this since one can never know what the future needs will be ;-)