How about being informative, interactive, intriguing and something I might come back to a few times. One of my first downloads for learning is an app called "The Elements" described by its author as "the universal catalog of everything you can drop on your foot: These are the elements, the building blocks of your world...". OK, that sounds good, but my recollection from chemistry class is that elements are boring.
Folks, this isn't boring. It is beautiful to begin with. Each of the elements (other than the gases) was filmed in a gorgeous 360 degree view. Apparently with 3D glasses (an extra) you can see them pop off the screen too. Not enough? You can spin an element and move them (use those iPad fingers). And it is a rich data mine: Want to know the melting point of Rubidium? 39.31 Celsius. Want to convert that to Fahrenheit? 102.76. Want to know what Rubidium is? Well one of its few applications is providing the purple color in fireworks. How about the crystal structure, atomic radius, and level or electronegativity? There at the TAP of my finger.
What I see in "The Elements" in my early exploration is a great window into making learning work for the learner. Sure, all of this information is also on the web but this gives me ease of use, a great look and feel, and links to all kinds of additional websites from within the program.
Finally, if you remember fondly, or have only heard about the Tom Lehrer song "The Elements", the app includes not only the song, but illustrates it with every one of the elements popping into the periodic table as the song goes through its 90 second tour of those things which fall on our toes.