Just thought I'd mention Incoherence, for those who haven't come across it ....
Plugins for iTunes (Win&Mac), Winamp, WMP and XMMS are available.Incoherence is an audio visualization tool that shows you the placement of sounds in the stereo field and the frequency spectrum, simultaneously. (High frequencies are toward the top, low frequencies are toward the bottom, and left and right correspond directly to the stereo field.)
This is not a very common approach to audio visualization, but we think it's intuitive -- we call it a "stereo field spectrum analyzer". Incoherence can help you to determine whether sounds will be distinct to a listener, or whether they are masked. Also it looks real pretty.