This StartupItem silences the Mac OS X startup chime, colloquially known as the "bong" sound, on Intel-based Macs. PPC-based Macs use another method for changing the startup chime's volume.
So why not just run 'sudo nvram SystemAudioVolume=" "' and be done with it? For some reason, my MacBook keeps resetting the chime volume automagically, and some online research hasn't come up with a reason why. Until a simpler solution is found, this should work fine.
(TinkerTool System can silence the startup chime, but it is shareware. StartupSound.Prefpane can do this too, but DebongIntel is a simpler solution if all you want is to silence the chime. You could always modify the script to set the volume to a non-silent level if you hate life.)
For more details, see the full README.