It used to be if a guy wanted the ability the mould an entire celestial sphere to suit his fancy he'd need to make a pact with some sort of all-powerful otherworldly being. Today all one must do is drop $.99 on Topia World Builder.
More of a toy than a proper game, Crescent Moon Games' is an impressively simple and simply impressive 3D planetary sandbox. Raise mountains and lower valleys with a touch of your fingertips, paint the land with herds of alien wildlife and watch nature takes its course.
There really isn't all that much to say about it. I've spent several hours poking about in Topia, granting animals life and then condemning them to watery graves, drawing crude sexual pictures across an entire hemisphere, and generally being the vengeful god I know I'd become if I ever ran into a pact-willing all-powerful otherworldly being.
Topia World Builder -- $.99 [iTunes]