What apps are still missing from the App Store?

When you look at Windows Phone or BlackBerry 10 it's easy to see the well-known iOS and Android apps that are missing from their respective app stores. Even with Android, you can look at popular iOS apps like Tweetbot, Fantastical, or Letterpress, not to mention every single one of Apple's apps like GarageBand, Keynote, and iTunes Store, and notice their absence from Google Play. With the iOS App Store, however, it can take a little while longer. The Google apps are mostly there, as is BBM, and many Microsoft apps as well. So what's still missing?

There are the obvious things, like Apple's missing iPad apps, and the apps Apple simply doesn't allow like porn and software that modifies the system like themes. Office for iPad, though, is a notable exception. So's Halo. There are smaller examples as well. Westjet, the airline I used just this week, has an Android app but no iOS app, at least not yet.

Are there any others? Any apps you've seen on other platforms, or any problem you've had that the App Store had no solution for? In other words, what apps are still missing from the App Store?

Review: You really ought to check out Catalyst Black for iOS
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Review: You really ought to check out Catalyst Black for iOS

Catalyst Black is an online multiplayer game that has teams compete against each other to score points and take down opponents. It has both sci-fi and fantasy elements since players can turn into large beasts to do additional damage.