Tablet and smartphone options
iOS keyboards have word completion built in
CoWriter Universal app for iOS (requires subscription)
iWordQ iOS app ($20)
Chromebook options
Read & Write for Google Chrome extension has a word prediction feature (Chromebooks)
CoWriter Universal (Chromebooks, Windows, Mac)
Computer options
Claro software has a word prediction feature (Chromebooks, Windows, Mac, iOS)
SoothSayer is available for Windows
Free word prediction options
Lightkey can be added to Windows devices