A pulse detonation nuclear-drive was concieved of a while back, purely in abstract, of course. The theory was based on the concept of setting off progressively larger nukes behind a very substantial blast plate at a rate of several per second. I think is was called the Dedalus Engine, but am not certain. The thrust would come from the radiation emitted by the blasts and not the actual blast, since there would be none in space, and so the accleration would be mild, but almost limitless in terms of a top speed. Seems like an interesting idea, given their ability to safely shield the ship and crew from nuclear blasts with unheard-of yields. It's doubtful this idea will come to pass, however, as it's a bit overly dramatic and will probably be eclipsed by something more practical sooner or later. Solar sails function under the same basic principal, but without an internal fuel-source, would lose energy as distance to the sun increases.
PS: What's IIRC stand for?
