I'm sad that the idea hasn't made it to other operating systems, with a system level API to allow for setting volumes independently on sound streams in different apps (an app could expose any arbitrary number of sound streams - most apps would likely have just 1, but browsers could have one per tab).
In practice it depends on (at least) your volume manager app(let) so you don't see it on say XFCE.
An app called Jack OS X does the job on Mac: http://www.jackosx.com/
That panel on XP (and before) only permitted per-device volume control.
For reference see [1] at around 04:00 and Larry Osterman's posts from September and December 2005 [2][3].
[1]: http://channel9.msdn.com/Blogs/scobleizer/Steve-Ball-Learnin...
[2]: http://blogs.msdn.com/b/larryosterman/archive/2005/09/19/471...
[3]: http://blogs.msdn.com/b/larryosterman/archive/2005/12/15/504...
It's also a complete pain in the arse to get running on Linux, due to the ALSA+Pulse stack being in the way.
JACK is pretty complex, but infinitely useful; one of those things I end up installing long before I decide I actually need it.