The newer versions of VirtualDub include a very cool audio filter called "center cut". It is a vocal remover type filter, but unlike most, it manages to produce stereo output. If any C++ programmer has time, could you please make this into a Winamp 2 plugin? I briefly looked at VirtualDub's source, and it didn't look trivial to pull out this bit of code, but hopefully someone will be able to figure it out better than myself (I don't know much C++ anyway).

Or are there already other Winamp plugins using a similar method?

I forgot, the filter also has a mono center output as well. It would be nice if both effects could be put in the filter, selectable from the dropdown list in the DSP/Effect area of Winamp's preferences.

Here's a short clip of how the side output sounds (from M2M's song "Smiling Face"). If you want to try this with your music, make sure you have a song in WAV format. Load up a video in VirtualDub that's at least as long as your song (AVI or MPG will work) and select "WAV audio..." from the Audio menu. After you've loaded your WAV, select "Full processing mode", and then "Use advanced filtering" from the Audio menu. Select "Filters..." from the Audio menu, and arrange the filters like in this screenshot. You can hear the audio by pressing the output preview button on VirtualDub's bottom toolbar (3rd button from the left), or you can save the result to a WAV file by choosing "Save WAV..." from the File menu.

If you want to hear the center output, switch the "output" and "discard" filters (click on the arrows to select them, press the delete key on your keyboard, and then drag new arrows between the desired pins).

NOTE: The latest version of VirtualDub should be used, as there were some bugs with the audio filtering in older versions.