It was 50 years ago today that Alan Turing passed away. He is indisputably one of the most influential thinkers of the 20th Century, who's life included such accomplishments as the development of the Church-Turing Thesis and the Turing Test. He is considered the father of the modern computer and cognitive computationalism. He was also the guy who successfully hacked the Nazi's Enigma code during WWII -- one the first ever real-world situations in which a computer was utilized.
For all his successes he led a very tragic life, living at a time when homosexuality was considered both a crime and a mental disorder. At one point he had to take estrogen to curb his libido. He eventually killed himself with an apple laced with cyanide.
The BBC has a good retrospective of his life and accomplishments. Here's a great site dedicated to Alan Turing.