Julius and Ethel Rosenberg, executed for espionage 61 years ago today; June 19, 1953
They actually gave the Soviets the proximity fuse (which is a huge leap in anti air). Through Venona we were reading their codes and knew of them. Their handler has since written a book on it confirming they were spies. The atomic component they gave the Soviets was rather minor but still, used to make the atomic bomb. After Venona was declassified it is difficult to say they were innocent because, well, you can read the messages from their handler with their confirmed code names.
* The trial judge agreed on the death penalty before the trial began. An eternal blot on American justice.