Start the Screensaver
Disclaimer: This is an old post. Content may be out of date.
The screensaver engine on a Mac can be invoked from the command line. This can be useful for locking a Mac remotely if its security settings are configured to require a password after waking, but without needing to put the Mac fully to sleep:
open -a /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/ScreenSaverEngine.app