There were nuclear-powered engines developed for Apollo abandoned for political reasons that are quite a bit more environmentally friendly that the long series of thermonuclear warheads popping behind a ship
While not the best fuel it could've very easily made do with water.
Considering we can't launch a simple probe though without an anti-nuclear rally at KSC though I don't suspect it'll ever become popular again, at least not in the Western world, even if it did have the potential to shoot us to Mars in weeks instead of months or years, regardless of relative orbital positions. (I award myself the run-on sentence of the week award)
There's a really good wiki entry on it. It leaves out a few interesting things, mostly more recent technological advances, but it's a great read even without being up to date. Not it's fault for being out of date either; I got it directly from a nuclear engineer familiar with the field, so it's rather hard to get any closer to the story then that.