Alan Turing

Alan Turing was a British mathematician and computer scientist whose accomplishments left very positive marks in history and some of which have made important impacts on today's world.

He invented the Turing machine, which was the basis of a computer in that it would be able to carry out numerous, specific tasks once provided the proper program.  The Turing machine's logical capabilities defined what an algorithm is. Computer science wouldn't exist the way it does today without Turing.

In addition to his contribution to the computer world, Turing was also an expert codebreaker.  In World War II, he and another mathematician, Gordon Welchman, managed to decode messages enciphered in the "Enigma" machine by the Germans.  He was able to do so because of the British "Bombe," which he designed alongside Welchman.

Despite this man's amazing feats, he was arrested for homosexual acts in 1952.  Just two years later in 1954, Turing was found dead in his home by cyanide poisoning, most likely via the half-eaten apple that was found with his body.  It was deemed a suicide.
As unfortunate as this was, Turing's incredible achievements will forever be remembered, and computers likely would not be the way they are in modern times if not for Turing's efforts and innovative mind.