It used to be possible to silence the loud "Bong" sound that all Macs make on startup by simply hitting the "Mute" key on the keyboard before starting the computer. This no longer works on the Intel iMacs, nor does adjusting any sound settings in the System Preferences.

However it turns out there is a Preference Pane you can install that will give you full control over the irritating chime. Download StartupSound.prefPane from Arcana Research, install it, and adjust the new Startup Sound setting however you like.

Many thanks to Alex Brie, of Hack The Day for this tip.