The Man Behind the Face
Turing laid out some of the most notable theoretical foundations on which programming is built. He is credited with the formal outlining of central concepts of the field, such as computing and algorithms. He also worked with Von Neumann to develop early philosophies and ideas on artificial intelligence.
While the famous Turing Machine was not strictly a computer, it was a large stepping stone in the development of the devices; specifically, in outlining the limits of what mechanical computational devices could achieve.
Check out Alan Turing's Wikipedia page for a thorough description of his accomplishments and contributions.