Ever wish there was a notch of volume between zero and one? Simply press the “volume down” key until there are no dots showing, and then hit the “mute” key.
The volume icon will display sound again. Even though the volume measurement bar remains blank, you should be able to hear the faintest of sound (you might need to plug in a pair of headphones).
This works at least in 10.6 Snow Leopard; not sure about Leopard or Tiger.
[Editor’s note: I verified this on my 2010 15-inch MacBook Pro. I’m not sure how useful it’ll be, but it’s there for people who want it.]