Antimatter annihilation is the best source of energy per weight. However, antimatter is not readily available anywhere within reachable distance, it's insanely difficult to produce and contain.
However, if a society with our current level of technology (or just slightly more advanced) discovered a source of antimatter, how could they use it?
Just having a big chunk of antimatter floating in space. I doubt that we could harvest it. Maybe we could bombard it with particles to make it glow, and then harvest that energy, but I guess it wouldn't be much different than harvesting solar energy directly.
If the first version cannot be used at all, let's make it much easier. We get the antimatter in nice self-contained packages (recovered from an alien shipwreck, or trading for it with a different civilization, it doesn't matter). In such a container, the size of a car battery, there are a few grams of antimatter, electromagnetically kept away from annihilating the walls of the container. There is a valve which can be opened to release a thin stream of antimatter, but once out, it is of course free to annihilate itself with any matter, including the container itself, as it is only protected from the inside. So it should either be opened in vacuum or the antimatter otherwise used up or guided by various means to its intended place to be annihilated.
How can such a source of antimatter provide useful work? Would it just be used to heat water which will drive steam turbines, like in a nuclear power plant? Or are there much more effective ways to extract useful work out of it? How could it be used to propel spacecraft?
How could we with our level of technology utilize such a source of antimatter? Besides threatening to use it as a weapon by breaking open the container.