If you are interested in physics, you certainly know this website: Hyperphysics. It gives a rather complete overview of laws and equations of the physical world. Even if I prefer to learn physics from storytellers like Feynman or Penrose, Hyperphysics is really handy when you have to gather your recollections in physics and have a fast overview of this very very vast field.
I have recently discovered that they offer an IPhone app; given the quality of the website, I did not hesitate and I tortured my credit card to get it. And I was disappointed: the quality of the images and equations is poor. You cannot zoom them, you have a bad ergonomy of the interaction. There is, all in all, not use of the huge potential of the smartphone and it is a pity, because you could gain a lot from this physics encyclopediae. My advice: as long as there is no improvement of the interface, do not buy it! (But dupport the website, they deserve it!).