6 Differences: The sequel to
5 Differences, and it's just as lovely as the first. In fact, it might actually be even more lovely than the first, since there are nifty telescope images used for the sky in several levels. And some levels are mirror-imaged for added niftiness. :)
(It claims to be more difficult than the first, but I thought it was a smidge easier, i.e. I didn't get terribly stuck at all this time.)