OS 10.3 and 10.4 provide the ability to lock your screen without enabling a screensaver (like the “lock computer” option on Windows 2000/XP/2003).
Open Keychain Access (located at /Applications/Utilities) and choose the following (depending upon your OS):
10.3: View menu -> Show status in Menu Bar
10.4: Keychain menu -> Preferences; then select “Show status in Menu Bar”
For both 10.3. and 10.4 the Keychain menu will now be avialable and the option “lock screen” can be chosen.
Very useful for servers where you do not want to place any further load from the GUI on the system.