John McCarthy truly was a visionary, coining the term artificial intelligence way back in 1955, and with the Dartmouth Conference of 1956, McCarthy established the field of Artificial Intelligence, together with Marvin Minsky, Claude Shannon, and Nathan Rochester. This wasn't his only accomplishment, however.
McCarthy also created the LISP programming language, the second oldest high-level programming language in the word (FORTRAN is the oldest). The LISP programming language was officially proposed in McCarty's research paper 'Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I', which is available for free online. As part of further improving LISP, McCarthy also invented garbage collection.
He further contributed to many other aspects of technology, such as timesharing. While Bob Bemer was the first to propose a time-sharing mechanism for computers, McCarthy was the first to develop a working time-sharing system on an IBM 704; the Compatible Time-Sharing System, grew out of McCarthy's work, and was the precursor to MULTICS, which in itself was the precursor to UNIX.
As said, he contributed to many more fields within information technology in a time when the field was just starting to get a solid foothold in the world of science. I don't know about you, but October 2011 is more and more starting to look like The Month Computing Died.
Not being a programmer myself, I stole this from HackerNews reader pjscott.
(will-be 'john-mccarthy (modifier 'missed 'dearly))