I remember reading a thread about this before but can't find it now so will post afresh.

Kites are no longer just for leisurely days at the beach. A promising enviro-enterprise to watch, KiteShip is a wind-power-tech company proposing the use of massive sea kites to reduce the amount of fuel used by commercial freighters. And recently, company executives have announced a potential partnership with four shipping companies to build a $2 million, 13,000-square-foot kite to help haul ships as large as 400 feet long. The huge kite would cut fuel costs drastically, making the gas-guzzling commercial shipping industry a little greener.

http://www.kiteship.com/