iSpazz is a plugin that you install by uncompressing the .zip file and moving the plugin to ~/Library/iTunes/iTunes Plug-ins. If you are new to Mac OS X, the ~ stands for your home folder. So, on my computer where I would copy the iTunes Plugin would be Nick/Library/iTunes/iTunes Plug-ins. It is an extremely cool plugin because your keyboard will light up in sync with your currently playing track. Pretty cool!
Once you add iSpazz, in iTunes go to View>Visualizer>iSpazz. Play a track, and prepare to party. If you are a DJ, this would be extremely cool, especially if you are using a Mac to power your media.
Image Source: OSXDaily