There was a different form of nuclear propulsion, tested in labs if not in space - look up NERVA. Rather less insane it used a more stable nuclear reactor to heat gas which could expand out the back to provide propulsion IIRC. It was dropped due to the risk of launch pad fires or other failures spreading uranium all over the area.
I knew I shouldn't have said only when I was typing that and that there were others I was forgetting. I forgot about NERVA and about Solar Sails, there's probably others I also forgot. They're still all fuel or time limited to non-relativistic speeds, currently. If someone knows of one that isn't, even a purely theoretical one, I'd love to hear about it.