When you boot your Mac, the sound it makes when it starts is pretty loud and annoying, especially if you are in a quite place like a library or some other place in public. If you want to control the volume or get rid of the sound completely, there is an application that allows you to do this. It's called StartupSound.prefPane and you can find it here: StartupSound.prefPane.

Once you install it, you get a new option in your preferences called Startup Sound. Clicking on it, you can adjust the volume of the sound made when your computer boots, or you can click the "Mute" checkbox to get rid of the sound all together.

This application is free, and it works with Leopard as well as Tiger. I found (and I think others have experienced this too), that the first time you boot after installing this, it doesn't work. But it does work on the boots after that.