iOS 8 Only:
Took Apple seven years to allow alternate keyboards on iOS, but the wait is over with this week's release of iOS 8. My favorite, which I've used for years on my Android phones, is Swype ($0.99) and one of the main reasons I have an Android phone. There are several others including Swiftkey (free, I love free) and Fleksy ($0.99). Read about all the above and more at this CNET article.