Synaesthesia. It presents an image with the horizontal axis being stereo and the vertical axis being frequency. Images can directly correspond to individual sounds, and so I can see sounds on the screen as I'm hearing them. The program truly deserves the name Synaesthesia.
Until now, the only publicly released Windows version was an old port based on Synaesthesia 1.2. The current version of Synaesthesia is 2.4, and there are many new features since 1.2. A long time ago I started working on a port of Synaesthesia as a Winamp plugin. However,
I learned that GPL licensed code (Synaesthesia) can't be used in a plugin for an application that has an incompatible license (Winamp). I just created a new port based on Synaesthesia 2.4, the 1.2 Windows port and some of my code. You can download it from Dropbox. Further information is available within "Synaesthesia 2.4 Windows port readme.txt" in that zip file.
This is just a quick port I created today. There may be bugs. Please don't bother the original authors of Synaesthesia and the 1.2 port. If you run into problems, leave a comment here.