I recently came across an article that mentioned the nuclear powered engines for rockets that NASA was developing in the 1960's and '70's. I have to admit I had no idea that had been going on, although obviously a range of space probes are nuclear powered. A bit different from the rockets that provide the lift though. What is really amazing is how much more powerful and efficient they were than the conventional chemical rockets we are using today. They would have allowed us to reach Mars in a few months and Saturn in less time than it takes to get to Mars today. If man had chosen to go down this route we would almost certainly have had a manned visit to Mars years ago. It was certainly planned.
I think it is interesting to consider the differences that manned missions to Mars, and the technology spin-offs from that, might have made to our history over the last few decades. A path that we chose not to take. Anyway, NASA being generally quite open these days, you can download a report that they commissioned into the program (after its end) here. There is also a nice summary here and a longer article here.