New Scientist provides a list of 10 possible propulsion technologies for deep space technologies, ranging from the ion thruster (plausibility: just a few years away) to wormholes (plausibility: almost certainly impossible.)
It’s a great list. My sentimental favorite has got to be the Bussard ramjet — a fusion drive that runs on hydrogen collected in deep space along the spacecraft’s route, ramming fuel into the ship’s maw using a vast scoop. Cool! But I think the most realistic choices, for interstellar travel, anyway, are the ion drive and beam-driven space sail technology.
Something like a fusion drive would be great for moving big Starship-Enterprise style craft around the solar system, but I think that’s as far as we’ll want to go with what I’m going to call Macro Human Space Travel (the original term for this idea was “manned spacecraft.”) Nanotechnology and artificial intelligence will enable us to explore interstellar, and perhaps eventually intergalactic, space much more efficiently using very small space vehicles.
With sufficiently advanced nanotechnology, we could fit all the equipment required to explore a star system and build several new ships to continue the mission, as well as the computing power required to run an interesting virtual world on the ship, along with a crew of several thousand downloaded explorers on something the size of an iPhone — or smaller. The real limiting factor would be how small we could make an effective ion drive or how accurately we could hit such a small target with a propulsion beam across vast distances.