Oleg Penkovsky (1919-1963) was a colonel in Soviet military intelligence, the GRU. He became an ideological opponent of the regime, and volunteered his services to the U.S. Central Intelligence Agency and British Secret Intelligence Service. He remained in place, providing secret information, until he was discovered and shot.
Penkovsky is often considered the most important Western spy of the Cold War. He had extensive access both to information on Soviet military technology and strategic thinking. In particular, he provided substantial information on the Soviet ballistic missile programs, such that the Soviets triggered the Cuban missile crisis as a result of the imbalance between U.S. programs and what the U.S. now knew of the Soviet deficiencies.
The main code word for his material was IRONBARK, which can be searched in the online CIA Freedom of Information Act reading room.