Galactica is an interactive light show, a relaxing app that lets you manipulate spacey visual effects to the sounds of ethereal background music.
As is often the case for apps like this (i.e., with arguably zero "usefulness") the value of Galactica comes down to how well it's designed and executed. Fortunately, Galactica makes excellent use of the touch screen, and the quality of the music and the app's cosmic "fireworks" are consistently good. The interface encourages playfulness, with a simple flick to send a stream of light shooting across the screen, a double-tap for an explosion, and a cool gravity effect that gathers light with multitouch ("From two touches, release to one touch for a single attractor.") Galactica even includes a short, unobtrusive tutorial.
This app isn't for everyone (especially since it's not free), but for anyone who wants a fun way to space out and unwind--or to keep a young kid entertained--Galactica is a pretty slick package.