PulseAudio, I already get that for free :) The default KMix app in KDE presents app volume sliders as well as the Master audio slider in the tasktray icon. One of my absolute favorite linux features, especially since I pass through my Xbox and I can control it too.
Oh, Windows does it too--but that's per app, not per tab. Apparently quite a few people like to listen to music on youtube while playing my game with the volume off :)
