Space based solar power systems that beam power back to terrestrial receivers have been discussed for decades. However, I've always had a nagging concern that such a system, once built, could be quickly repurposed as a weapon, and so no country would permit another country to launch one.
I know the power density on the ground wouldn't be high enough for a dramatic, fry-a-city-in-ten-seconds kind of weapon. In fact, Wikipedia cites expected levels of $23mW/cm^2$, which is only about twice the OSHA workplace limits. However, if you follow Wikipedia's reference to its source, on page 14 you'll find that this limit is designed to "avoid potential microwave interference with the D and F layers of the atmosphere."
For those who don't care about the ionosphere (which probably includes those hoping to create a weapon) it would likely be easy to design a transmission antenna that would usually limit itself to this power density, but could easily be focused to increase it by perhaps an order of magnitude. $250mW/cm^2$ would be about twice the solar constant, and would impart significant amounts of heat to anyone in the beam. Given that the beam could cover a large city, and add in infrastructure damage, and that's a pretty significant attack.
Is this a legitimate concern? Are designers of potential space solar power systems coming up with ways to eliminate the ability to repurpose them into weapons?
Edit: Most proposals put the transmitting satellite in geosynchronous orbit, so that it will always be above its dedicated receiving station. This puts the satellite out of the range of current anti-satellite weapons, and could even make it difficult to destroy in the future; it would be so large, and could be built from redundant sections, so that punching holes through it wouldn't significantly harm it.
I also understand the comparison to nuclear weapons. However, the use of nuclear weapons is such a bright line that the attacker would likely receive worldwide condemnation. Nor can you use a "little bit" of nuclear weapons, which also keeps that line bright. However, a divertable microwave beam might be easier to use briefly, and even deniably ("Ooops!"). Imagine a country briefly toasting one of its surrounding countries' capitals, and then immediately apologizing. The effect could be drastic, and could seriously destabilize the attacked country.