A Solar Steamer is a reusable solar powered rocket that uses super heated steam as a reaction mass.
The basic design consists of a large, roughly parabolic mirror, that focuses the sunshine down onto a steam chamber.
The steam chamber boils water and passes the resultant steam through a rocket nozzle giving a resultant exhaust velocity of about 1900 km/s (i.e. an ISP of 190s).
By most standards, the performance is very mediocre indeed! Normal rockets hae exhaust velocities of upto 4500 km/s. Nevertherless, there are very serious proposals to use this design in space.
One common counter-idea is that it might be better to electrolyse the water and burn the hydrogen/oxygen to get more thrust. This works. However, the power generation equipment needed to do this turns out to be very heavy, and this blocks its probable use in many circumstances.
So, the Solar Steamer would be very useful for moving between the planets, beyond Jupiter, or to Mars or the asteroid belt; basically wherever a source of water can be found at the destination, you can go basically anywhere. Water is believed to be quite common in the Solar System; in particular: at comets, the asteroid belt, possibly the Lunar pole, and Deimos all seem to have reasonably easily accessible water; for orbital mechanics reasons these should be outside any strong gravity (this means that the Earth and Mars water is a poor choice for this application).
The return trip from these places is 'mostly downhill' (i.e. requires little delta-v/propellent); and it would be very possible to return water to LEO very cheaply. In contrast launching fuel from the Earth's surface would be very expensive. (See Costs of Launching to Space).
Many people reading this will think, so what? What good is this to me? Well.... It's looking like Space Tourism is likely to be here big time in about 15 years time; remarkably a trip to space looks to be heading to under $10k longterm; so a lot of people are going to go into space; specifically earth orbit, but the Moon and Mars as well. It turns out that earth orbit is nearer to Mars moons and Near Earth Objects in speed terms than the earth's surface, so it makes sense to import stuff from other places as much as possible; and the Solar Steamer gives us that capability.
Because water is multipurpose it's useful as a propellent, or as a radiation, meteorite shield, for building things from it (ice), and it's easy to store, wash with and many industrial processes use it; you can even drink it (if it still seems clean enough for you)! Space Mining water is almost certainly going to be big business. The main use of water will probably be to send extra-terrestial resources back to LEO for hotels and such like.
The Solar Steamer will bring the resources of the solar system into reach of Mankind. Once there, other technologies will take over; or atleast this is possibly the nearest thing to a consensus view right now.
Another similar technology is the Nuclear Steamer, which uses nuclear power instead of solar power.