Barbarian Software Group's Magnetosphere is an awesome particle-based visualizer plugin for Apple's iTunes software. Once you've installed it, open iTunes and select "View" then "Visualizer" then "Magnetosphere" from drop-down menu. From there just play some music and activate the visulizer (View, Show Visualizer) and watch as your screen explodes with fireworks and streaming ribbons, all flowing to the rythm of your music. It's quite enthralling. You can adjust the number of particles by pressing the A (add) or S (subtract) keys, change cycles with the C key, or access the help menu by pressing ?. And yes, it's free.