The best part of Winamp visualisations, was that you could code/customise your own visualisations [1]. With a few equations and some variables in a limited context (frame, loop, pixel) you could create wild things. It was the first time I was exposed to this type of programming of simple "shaders".
[1] http://www.geisswerks.com/milkdrop/milkdrop_preset_authoring...