If you've seen the Star Wars prequels or read any science fiction, the idea of a "solar sail" probably isn't too unusual to you. These as-yet-undeveloped technical marvels capture the pressure created by the winds flowing off of stars to (theoretically) move spaceships across a system without engines or fuel. However, the technology could be closer to reality than we think …
An electric solar sail works more or less the same way, except without the sail: all you need are a bunch of wires.
The sun is constantly spewing out a stream of photons, along with charged particles called the solar wind. The solar wind is not nearly as strong as wind here on Earth, but if you put a big enough sail out into it (we're talking like miles across) and aren't in a hurry, you can use it to propel spacecraft, harvesting energy from all the photons and charged particles bouncing off of the sail.
Since you've got charged particles to work with in addition to photons, it's possible to just ditch the sail part of a solar sail completely and rely instead on an electric field. All you need to make an electric solar sail like this is an array of about a hundred 10-mile-long, 25-micron-thick conductive wires. After the wires are charged up to 20,000 volts or so using a solar-powered electron gun on the spacecraft that they're attached to, they generate an electric field that extends out to about a hundred meters surrounding the wire. Charged particles bounce off this field like it's a solid surface, transferring their momentum to the wires, which in turn propel the spacecraft.
Relying on charged particles like this gives you about about 5,000 times less energy to work with than if you were to use a traditional solar sail, but on the upside, the wires are very easy to manage and barely weigh anything, making the system efficient overall. Typical acceleration of an electric solar sail might be in the range of one single millimeter per second², but after about a year, you'd be cruising along at a blistering 30 kilometers every second. This could get you to Pluto in five years or less, without needing an engine, fuel, or wasting more than the absolute minimum amount of mass on your propulsion payload.
Wanna hear something cool? Someone came up with this electric solar sail idea in 2008, but it's going to be tested, for real, on a sattelite from Europe next year.
The future is coming, y'all. Believe that.