one of my favorite apps is the starmap. basically a map of the night sky (specific to your area, as determined by the GPS of your phone), from there you can touch any star in the sky and it'll give you info about it, it'll give you info about the constellations, it'll let you zoom, and it'll let you time compress, so you can see what the sky will look like in 1000 years. it's a very cool app.
i also like x-plane. it's a pretty decent flight sim for the iphone. it absolutely murders battery life though.