LED Spectrum Analyser 2.0 for iTunes is a colorful visualizer for iTunes that emulates a LED Spectrum Analyzer, with lots of customization available of layout, colors, meter ballistics and much more. A complete rewrite of a similar earlier release for SoundJam, with many new features.
LED Spectrum Analyser 2.0 for iTunes weighs in at just 300K and is free.
More info and download link here.