These programs can keep unauthorized users from accessing your computer in your absence.
DesktopShield 2000 Secures your PC when you're not using it. Freeware. (http://www.dilawri.com/software/sysprotect/)
Workstation Lock Password-protects your system at startup without using a screensaver. Shareware. (http://posum.com)
SuperLock Pro and SuperLock Lite Prevents unauthorized access to your Macintosh by recording any break-in attempts, sounding an alarm, automatically going into sleep mode, and stopping someone from rebooting your computer. Shareware. (http://www.trivectus.com/)