StartNinja 1.0 - Silence the startup chime on Mac OS X 10.7 Lion.. (Free): "
StartNinja will silence the Mac OS X Lion start up chime with StartNinja. It is a free and simple desktop utility that silences the start up chime and will resume your previous sound level after logging in.
Mac OS X 10.7 or later.
(Via MacUpdate - Mac OS X.)