Apps
The first App I made is called The Elements: A Visual Exploration. It's very nice. You can buy it on the Apple App Store. It's more expensive than the typical free app, but it's worth it. No, it's not a crappy app like all those others you've seen. Really, you should try it if you have an iPad. (It's somewhat less compelling on an iPhone, so I won't say for sure that you'll be delighted with that version.) Currently there are no versions for Android, sorry.
The App form of Molecules is quite remarkable. Buy it here.
The Elements in Action is a sort of add-on app for The Elements: A Visual Exploration. It contains one very nice video for each element, shot by my partner Max Whitby and me. The idea is to demonstrate one interesting, characteristic property of each element. The home page of the app is a pretty cool periodic table where each square shows a bit of the video running. If you have both The Elements and The Elements in Action, they work with each other, allowing you to switch back and forth easily.
The Elements Flashcards is a free app that helps you memorize the names, atomic numbers, and symbols for the most common elements (or all of them, if you're that into it). Personally I think there's little or no value in memorizing this stuff, but lots of students need to do it for a test, and the available flash cards apps seemed so horrible to me that I thought we should make a better one. It's got a regular flash card mode, and a clever "place the element" mode that also teaches you where each element is found in the periodic table.