In 1958, Project Orion was formed at General Atomics (a defense contractor), seeking to design spacecraft powered by nuclear pulses -- in other words, bombs. The project designed incredibly large vessels, with theoretical sizes ranging up to 400 meters in diameter and weighing 8,000,000 tons. The ships were intended to travel to Saturn and Venus carrying large crews -- but the project ended in the early 1960's without building a full-scale vehicle.

The project involved, among others, physicist Freeman Dyson. Nearly forty years after the project was canceled, you can listen to Dyson's son George describe Project Orion in a TED talk from 2002. He includes a variety of insider info, and it's fascinating stuff (if you can get past his somewhat haphazard delivery):

George Dyson went on to write Project Orion: The True Story of the Atomic Spaceship. You can also learn more about Project Orion on Wikipedia, or at this fan page.