ALARM is free and deviously simple -- setup is fast, and you can specify any MP3 file as the warning that sounds when trouble arrives (though an alarm sound comes in the installer if you don't have your own alternative).
Of course, ALARM relies on a thief walking away with your laptop lid still open -- as soon as you close the lid, the sound will stop as the system goes to sleep. But if your potential thief unplugs the laptop first and then starts to walk away with it, you'll hear the alarm sound. If you use your laptop in public and ever find yourself stepping away for a moment, ALARM is a small measure of admittedly incomplete security. [via ghacks]