I only recently heard about Project Pluto, and it's so crazy that it's hard to imagine anything so awesome could every be built.
The proposed use for nuclear-powered ramjets would be to power a cruise missile, called SLAM, for Supersonic Low Altitude Missile. In order to reach ramjet speed, it would be launched from the ground by a cluster of conventional rocket boosters. Once it reached cruising altitude and was far away from populated areas the nuclear reactor would be turned on. Since nuclear power gave it almost unlimited range, the missile could cruise in circles over the ocean until ordered 'down to the deck' for its supersonic dash to targets in the Soviet Union. Once powered up, the unshielded half-gigawatt nuclear reactor would emit highly lethal radiation in a large radius; such a vehicle could not possibly be human-piloted or reused. Indeed, some questioned whether a cruise missile derived from Project Pluto would need a warhead at all; the radiation from its engine, coupled with the shock wave that would be produced by flying at Mach 3 at treetop level, would have left a wide path of destruction wherever it went. The SLAM as proposed would carry a payload of many nuclear weapons to be dropped on multiple targets, making the cruise missile into an unmanned bomber. Contrary to some reports, the exhaust of the engine would not itself be highly radioactive.
On May 14, 1961, the world's first nuclear ramjet engine, "Tory-IIA," mounted on a railroad car, roared to life for just a few seconds. Three years later, "Tory-IIC" was run for five minutes at full power, producing 513 megawatts and the equivalent of over 35,000 pounds force (156 kN) thrust. But despite these and other successful tests the Pentagon, sponsor of the "Pluto project," had second thoughts; Intercontinental ballistic missile technology had proved to be more easily developed than previously thought, reducing the need for such highly capable cruise missiles. On July 1, 1964, seven years and six months after it was born, "Project Pluto" was canceled.
If you like that, you might also like the nuclear-powered long-range bomber.