AnySoftKeyboard - Android (f/w 1.5+) on-screen keyboard for multiple languages.
apole - iOS keyboard extension to see your calendar while messaging
Gboard - Google-powered keyboard with search, GIFs, emojis and more!
OftenType - Keyboard app with custom quick keys for often used texts.
FlorisBoard - An open-source keyboard for Android.
Mood Chat (iOS Keyboard) - Add background sounds to your voice messages