Automatic volume fader to quiet iTunes when a video plays

I usually play music when I’m using my computer to surf the web. And this being 2008, I often come across videos on YouTube (or other similar sites) that I want to watch. The problem is that I have to stop the music I’m playing in iTunes so that I can hear the sound from the video. I know that this is an easy task on a computer with a dedicated PAUSE button on the keyboard, but even so, the action of moving my finger over that button is just annoying video after video.

I just want an app that will automatically fade my music out as sound from my browser kicks in.

Does anybody know of an app that can do this? I would be forever grateful to the person who links me to an app or writes the program that does this exactly.