There has been speculation on solar sails. Solar sails are like a sail boat's sail, in space. It bounces protons sent out by the sun to propel itself forward(With an action, there's a reaction). Anyways, there have been various sails attempted to be built, and I believe a few have some what worked.
Recently, a small satellite was made, that uses a solar sail to propel itself. Electrically charged wires repel protons in space to push itself forward. Right now the satellite is only in Earth's magnetosphere getting tested upon.