There are many great STEM resources out there on the iPad. Personally, I am a bargain fan though so I am highlighting free resources. You don't get any better than free.
The first three are all created by Autodesk. I believe they must see these apps as an investment in their future employees because not only did they create engaging and visually stunning apps that really make your brain hurt and give them away, but they also provide free training for educators on their STEM website.
AutoDesk Digital STEAM Applied Mechanics teaches basic mechanical engineering principles through games in these five topics: Energy and Work, Forces, Loading, Power, and Mechanisms. Players learn these principals by playing games that apply them. I learned that my understanding of forces and loading are far superior to those of Power, Mechanisms, and Energy and Work. I tried playing each of the latter three games several times and each time I lasted less than seven seconds before the game was over and I lost.
Digital STEAM Measurement is an app where even the tutorial is pretty nifty. You click your way through each of the measurement topics and get a brief, entirely visual introduction to the various forms of measurement. Topics covered include: Linear Measurement, Perimeter, Area, Volume, Angular Measurement, Proportion, Weight, Liquid Volume, Temperature, Mulitple Variables, BTU, and Power. It looks wicked, awesome cool too!
Digital STEAM Visual Design teaches the language and basics of design. Players can wade to their knees in 3D design as well. The Elements explored are: Space, Point, Line, Shape, Form, Value, Texture, and Color. Principals covered are Balance, Emphasis, Rhythm, Unity, Proportion, Contrast, Continuation, and Economy.
DIY Nano HD helps students investigate the nanoscale through DIY projects. What is the nanoscale you ask?! (Insert maniacal, cartoon-villain laughter.. Moo haw haw..) Why the nanoscale is the scale of atoms and molecules...of course! DIY Nano has three main elements: Activities, Videos, and WhatIsNano.org. The activities are fun, well explained and organized, and are simple to do with most materials easily and inexpensively acquired.
NatureTap fuels the bio science lover's needs. It comes with a grid of various birds from all walks of life. You can see stellar photos of them and listen to their calls while reading an Audobon-esque description. Then you can test your new knowledge by playing games and completing puzzles. With a free registration you can also get the Spiders and Insects grid. For $0.99 you can learn more about mammals, reptiles & amphibians, and wildflowers with more areas slated for development.