SwiftKey is arguably the best third-party keyboard for Android, and has become significantly better since the release of version 3.0 a few weeks ago.
The latest update, version 18.104.22.1682, adds official Jelly Bean support (though the old version works fine on my Galaxy Nexus) and a number of lingering bug fixes, including:
– UI fixes including missing secondary characters
– Predictions not appearing in popups
The app costs $3.99 and is currently the most-downloaded paid app in the Canadian Play Store. There is a free one-month trial version as well, which I’d recommend to anyone unhappy with their phone’s current keyboard.