Vannevar Bush
Imagined the memex in 1945 and launched the dream of computing as a tool for thought.
Vannevar Bush was an American engineer who, as director of U.S. wartime science, coordinated the research that produced radar and the atomic bomb. But his lasting influence on computing came from an essay, not a machine. In As We May Think (1945) he described the memex: a desk that would store all of one's books and records and let the reader link them by associative trails, mimicking how the mind actually works.
No memex was ever built, yet the essay seeded the whole tradition of computing as a tool for thought. Engelbart read it as a young man and never forgot it; Ted Nelson cited it when he coined hypertext. The web's links are the memex's trails, finally made real.