The harpsichord and the clavichord!
Google recently released Google Scribe, an experiment on auto-completing words for the user in advance, instead of spell and grammar checking what he’s already written. Go try it for yourself.
I myself had some fun letting Google write stuff for me. Just start with an arbitrary word, and keep accepting whatever the first suggestion is. The UI slows down quite a lot after a dozen or so auto-completions, so sometimes you need to delete the last space and type it again to get some performance back.