According to Official Guinness Records,
A 420 m² (4,521 ft²) traction kite manufactured by KiteShip Corporation (USA) was used to propel an 8.5 tonne (18,740 lb) yacht near Sydney, New South Wales, Australia, on 6 December 2004, the largest kite ever used to pull a vehicle of any kind.
A traction kite is similar to the aerofoil kites flown by kiting enthusiasts, but on a much larger scale. In the context of yachting the kite is used to replace the spinnaker (although in this instance no other sails were flown in addition to the kite).
For a complete list of 2004 records, please visit 2004 Guinness Records in Australia.