If you have kids who like to use the computer, you might occasionally find that they like to crank the volume to Disaster Area-like levels. iPods have had a volume limiter for years and wouldn't it be great if your kids' iMac couldn't ever get too loud?
Turns out that this is possible with a little Terminal magic:
defaults write com.apple.soundpref VolumeLimit -float [value = 0.0 - 1.0]
The last time I tried this, I recall that a reboot might have been required. That might not be true in the very latest point releases of Snow Leopard.
I didn't know this existed until I filed a bug requesting it.