Zio is a nice little app ($.99 on iTunes) that produces some great visualizations. It comes with several loaded presets which can be reconfigured to your liking. This app allows you to change many settings, producing a huge array of visualizations. A really nice feature is the ability to make your visualizations sound-reactive. The quality of the visualizations is quite good. One thing however that is bothersome about this paid app is the banner ad that shows up when you first start using it. Fortunately, it turns off after about two minutes of use.