Forgive me for not finding the old thread (something along the lines of what do you want to see in X), but perhaps these guys were listening:
http://www.rogueamoeba.com/detour/
I've always wanted to be able to control the volume level of individual apps. Every now and then you come across a otherwise really good shareware game that is tied into the system sound control. So if you want to fire up iTunes in the background and play the game without sound, you're hosed. And the more obvious application would be to have iTunes playing while you're fragging away in Quake or something. It looks like Detour finally accomplishes this. From the screenshot, it looks like a system pref. I'm away from my Mac right now, but I can't wait to try this when I get home. Sweeet!!!!