Of his contributions to science, his most famous is probably the concept of using satellites at a geostationary orbit for telecom relay, which he first proposed in the 40's, and is now known as the Clarke Orbit in his honor.
From the article on CNN:
As a Royal Air Force officer during World War II, Clarke took part in the early development of radar. In a paper written for the radio journal "Wireless World" in 1945, he suggested that artificial satellites hovering in a fixed spot above Earth could be used to relay telecommunications signals across the globe.
He is widely credited with introducing the idea of the communications satellite, the first of which were launched in the early 1960s. But he never patented the idea, prompting a 1965 essay that he subtitled, "How I Lost a Billion Dollars in My Spare Time."
