The Planetary Society is planning to build and launch a solar sail by the end of next year thanks to $1 million from an anonymous donor.
Solar sails are basically sheets of extremely lightweight material that propel spacecraft by capturing the photons of the solar wind, much as nautical sails propel ships by harnessing the energy of terrestrial winds. Literally, solar sails ride on light. The acceleration, of course, is tiny, but it is constant. Over years, so the theory goes, that constant, tiny acceleration could develop impressive speed-- and, of course, the energy is free.