Apparently there are a number of free games available on the App Store. Here is the Kotaku link I found out about this through. They may not have a full, comprehensive list of what's free, but I did manage to pick up some games I've been wanting for a while.
In other news, Limbo is apparently on iOS now. If you never played it on the console, it is an action/platformer/puzzle/horror game with a delightfully eery aesthetic. I strongly recommend it.