ChemTouch - A wonderful, interactive periodic table*, with *loads* of info. Also comes with info o amino acids and nucleotides.
Cosmic - From the American Museum of Natural History, a mosaic of some astronomical photos, that make up a mosaic of Saturn.
Dinosaurs - again from the American Museum of Natural History, a big bunch of dinosaur/fossil photos that make up a T. Rex mosaic, and a load of info about a number of different dinosaurs. Like the above, but for dinosaurs.
Exoplanets - info on exoplanets. All of them.
Mars - A globe of Mars, with loads of info on terrain, our exploration of it and the people involved. (I've actually got the £0.69 HD version).
Molecules - 3D models of molecules you rotate and zoom in on. Comes with a few, and you can download more, or create your own!
TimeTree - defo a must have. Input any two living things, and find out when they shared a common ancestor, and the evidence for the estimate.
Alas, I couldn't find the above for Android.
Also, having helped (a bit. A very small bit) with the Meningitis Trust's Android and iPhone apps, I can't resist another chance to plug them. Loads of info on the signs and symptoms of meningitis (which can be confused with the flu or a hangover). The info in this app could save your life.
Please suggest any other good apps below!
*On that subject, I highly recommend "The Disappearing Spoon" by Sam Kean.