http://www.nullsoft.com/free/milkdrop

The coolest winamp visualization thingie ever.
Not only is it really swank looking, but you can make it behave like wallpaper!

Even cooler trick, there's a specific mask colour that it uses to overlay, if you make other things use that colour, you can make everything all psychodelic and kickass looking. (the default mask colour is R15 G0 B15, but you can change it)

I'd take a screenshot, but it doesn't work, given the way it operates. And yes, it sucks the life from my PC, and it's super impractical since there's no single text colour that will be easily distinguishable between all of them, but it's worth it 'cause it's damn cool. :D

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'Never confuse a single defeat with a final defeat.' -F. Scott Fitzgerald