I would like to know how to do this as well. I just recently purchased IK multimedia ARC room correction system and it works great! However, it only works as a plugin. This is fine for my mixing use, but I listen to most of my music through itunes!
I've heard of other ARC owners using a program called "audio hijack" to do this, but was curious of there are other hopefully free options out there. I'm downloading the audio hijack demo now, will post back the results:
http://rogueamoeba.com/audiohijackpro/