Once upon a time, these were called "sails," and until about 200 years ago they were the only way to get around on the high seas. Now, several shipping firms are experimenting with wind-assisted ships that use high-tech methods to harness wind power.
Commercial sailing vessels died out largely because the cost of manning them was far greater than that of the oil and coal needed to power engine-driven ships. However, with the rise in energy prices combined with technology that makes windpower more efficient while keeping manpower to a minimum, wind-assistance once again makes economic sense. Concern over pollution from fossil fuels makes windpower environmentally attractive as well.
A Danish team has developed what it calls a "SkySail," a large parachute-like kite that can pull a ship as large as an ocean-going freighter. A computer controls the SkySail, determining its optimum position at any given moment and making the appropriate adjustments. The team hopes to start deploying SkySails next year, though they are reportedly still working out the mechanics of launching and reeling in the SkySail.
"Windships" using metal sails are also under consideration. Again, computers control these sails, making constant adjustments for maximum power.
Any of these devices would be hybrids, as wind is not always the best power source at all times. Additionally, it could be years before these devices turn up on a commercial vessel simply because of expense and the shipping industry's conservative nature.
Sources: Roland Piquepaille's Tech Trends, NewScientist.com