I attended an amazing talk by David circa 2006 at Princeton where he demoed his probabilistic text entry system called Dasher [1,3]. I think it was made into an iOS app later.

I also fondly recall his videos on Gaussian processes - best explanation on GPs I ever came across. [2]

[1] https://www.inference.org.uk/dasher/

[2] https://www.youtube.com/watch?v=NegVuuHwa8Q

[3] https://dasher.acecentre.net/about/

EDIT: Fixed the youtube link in [2], added [3].