When I wrote the autoEqualizer plug-in for the Amarok (1.x) media player, to the best of my knowledge, there weren't any media players with this feature nor were there any plug-ins.
Recently, I came to know that the Samsung Galaxy S's media player looks to be having the Automatic Equalizer functionality. Not sure if this player is specific to Samsung Galaxy S or the Android Platform in general.
I'm not a music fanatic. While I see many of my friends and colleagues working, with their headphone on and music playing, for me, it is not at all possible. It is just the opposite. I can't work with music (or any kind of distracting sound) on.