Solar sails harness the pressure of the solar wind to propel spacecraft across the solar system essentially for free, without the need for engines or fuel. 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.
Someone only thought up this whole electric solar sail thing in 2008, but one wire is already scheduled to be tested out on a European satellite in 2012.