Mac users: Just one bar isn't the lowest you can go when it comes to volume. A hidden "super-quiet" level offers just a whisper when you want the quietest possible sound. Macworld explains how to get there:
Use the volume keys on your keyboard to manually set the volume to zero—this trick won't work if you use the slider in the menu bar's volume controller. To access the "super quiet" mode, press the Mute key on your keyboard. Bingo, you should now hear (very faint) sound from your speakers.
If you press Mute repeatedly, you'll see that the onscreen volume control still doesn't show a set volume level (none of the bars will be illuminated), but you'll definitely be able to hear the audio muting and unmuting, assuming you're in a relatively quiet location.
Okay, I admit that I'm not sure when this would be useful—when you just want to drive people with sensitive hearing nuts?—but still good to know.