RUDOLF "LUCY" ROESSLER
Allied espionage during World War II is often seen as revolving around "Ultra" the top-secret decryption of top-level Nazi Enigma codes by some of the world's earliest computers (designed by the brilliant Alan Turing, a man who is just barely well-known enough to not fit into this list).
This idea fails to take into account how suspicious the Soviet Union was of Britain's claim that they miraculously obtained a working Enigma machine, solved it using technologies and theories invented specifically for that purpose, and found that it was talking all about how Germany (at the time, at war with Britain and on good if edgy terms with Russia) was going to start some serious shit with this "Operation Barbarossa" thing.
For Russia, the key to the German battle plans was a man named "Lucy" — Rudolf Roessler, an anti-Fascist German publisher living in Lucerne who was in close contact with rebellious high-ranking members of the German General Staff since the beginning of the war. Working around the clock with his own Enigma machine and elements of the covert "Red Orchestra" Soviet radio espionage group, Roessler was able to pass along decoded communications to the Kremlin within six hours of interception -- four times as fast as Turing's computers, and almost as soon as front-line Wehrmacht units received their orders.
"Lucy's" greatest victory was the discovery of Operation Zitadelle, a summer offensive against the Kursk Salient which resulted in an overwhelming Russian victory and changed the tide and momentum of the war in the East.