Android: While you won't have trouble finding choices of new and interesting keyboards on Android, Slice offers one of the more seemingly futuristic approaches. You input characters via six circles and keep your fingers on the screen whenever you're typing.
Slice was designed to offer a typing solution that would allow you to avoid looking at the screen to see which characters you're hitting. Ultimately, this would result in faster typing because you just need to memorise the key locations.
The obvious downside is, of course, that you have a steep learning curve. The keyboard itself isn't intuitive, and it will take a lot of time to get started. It's also only available in landscape mode on Android tablets with 7-inch screens or larger (and it feels mildly cramped on a 7-inch screen).
There are many downsides, but it's one of those keyboard upgrades dedicated users will likely love after they put in the time to learn how it works. Slice also helps you learn too. You can play a Space Invaders typing game to help you get used to the unusual layout and functionality. While it's hard to imagine many people adopting Slice as a new typing paradigm just for tablets, if you can learn how it works quickly you'll likely appreciate the efficiency it offers.
Want to see how Slice works? Watch the video above for a demo.