PTHVolume

PTHVolume is an utility for Mac OS X 10.5 and later that is used to control the volume of your Mac OS X machine.

For most people the included Mac OS X widget is sufficient for controlling volume. There are however two situations in which PTHVolume is a much more useful alternative.
Multiple Output Device Control. The Mac OS X 10.5 volume widget works for changing the volume if you only have a single audio device connected to your computer. But what happens when you have multiple audio devices? This is where PTHVolume 2 excels, you can individually control the volume of multiple devices without opening up System Preferences.