This part of the Now Utilities suffers from the Claris syndrome, having been spun back into the Now Utilities, though Now still sells it as a separate product for $79 or so. I used it for a short time as a screensaver, but all it does (yes, we're now talking about what screensavers can do, other than just blank the screen, of course) is show a message or a picture to protect your screen from burn in. Boring. Its main function, however, is to prevent your spouse or mean, nasty, ugly industrial spies from looking at what's on your Mac. Screen Locker achieves this by password protection (we'll have to wait a while before voice or handwriting recognition become part of standard security measures).
Unlike most password protection systems, though, Screen Locker's is carefully thought out. The dialog box that lets you define your password has six text entry fields, one for the old password, one for the new password, one for confirming the new password (you only see bullets, not the letters you're typing), and three more for "backdoors." The backdoors are a unique part of the password protection that will help forgetful users. You can set the backdoor field types, so they can be the names of your three children or your three favorite Beatles songs, or whatever you're guaranteed to remember. Now actually suggests the names of your last three girlfriends or boyfriends - I wonder what that says about who they see as their potential users :-). The trick is that if you forget your password, you can enter all three backdoors and get back into the system. Another way of using them in a more corporate setting would be to let the system administrator set the backdoors in case you forgot your password.
Screen Locker alone is not the end all to security, if only because someone can always reboot your machine with a floppy. If you don't have the programmer's switch or MacsBug installed, and you lock the floppy drive (there are a couple of hardware devices that do this), then Screen Locker could be effective since it has an option to run at startup. Simply make it the first INIT to run by renaming it and it will become difficult to break in. Screen Locker protects itself, so even if someone can get to the Finder, they can't throw out or disable Screen Locker if they don't know the password. Finally, I was pleased to discover that Screen Locker and After Dark (a screensaver that does a lot, entertainment-wise :-)) coexist happily. After Dark has a password feature as well, but it's not nearly so complete as Screen Locker's. When the two of them are running, Screen Locker first blanks the screen, then After Dark draws on top of Screen Locker. If you need a small level of security, Screen Locker will provide it quickly, easily, and cheaply. If you need more security than Screen Locker provides, go all the way up to one of the serious security packages.