Many other keyboards are available, designed with features for improving your device’s usability or helping users with special needs, ranging from Klingon keyboards to keyboards with large keys and letters for users with visual or manual dexterity issues (such as small children). I expect we’ll see more types of keyboards in the months ahead, and there’s considerable room for improvement and innovation.
The third-party keyboard I personally prefer is Smile Software’s $4.99 TextExpander touch. It’s designed to go beyond Apple’s built-in shortcuts feature, allowing you to use the same shortcuts on both iOS and OS X. I use the keyboard for special characters, as well as for inserting text snippets that save me keystrokes and time. (You can purchase the TextExpander 3 + custom keyboard app from the App Store on your iOS device.)