How do you silence the startup chime on a Mac?

Check out the StarupSound prefpane. It allows you to control the volume of and mute the chime.

Note: I have not used its on leopard yet, so I'm not sure if it works. But I can confirm it works in 10.4

Edit: I forgot to mention that you could just hold the mute button on you're keyboard when you hit the power button to disable the sound for that boot.

This looks like what you want:

StartupSound.prefPane

It adds a pref pane that you can use to disable the startup sound.

I use a System Preferences plugin called StartupSound.prefPane . Google around for it. You can set the volume.