Inspired by:
http://dry.ly/2010/10/17/create-a-music-visualizer-in-7-steps-with-ruby/

Requirements:
Ruby-Processing (comes with jruby, processing, and minim)
Soundflower
http://code.google.com/p/soundflower/

MacOsX Setup:
install jruby
sudo gem install ruby-processing
install soundflower via pkg
launch Soundflowerbed
SystemPreferences > Sound > Input
  set sound input to Soundflower
If you have any issues, try playing with sound settings also in Audio MIDI Setup

Run:
rp5 run run_visualizer.rb