The US  Department of Homeland Security (DHS) has announced the implementation of the Electronic System for Travel Authorization (ESTA), which began to accept voluntary ESTA applications on August 1, 2008.

According to http://travel.state.gov,  ESTA is a new fully automated, electronic system for screening passengers before they begin travel to the United States under the Visa Waiver Program (VWP). ESTA applications may be submitted at any time prior to travel to the United States, and VWP travelers are encouraged to apply for authorization as soon as they begin to plan a trip to the United States.

Effective January 12, 2009, all VWP travelers will be required to obtain an electronic travel authorization prior to boarding a carrier to travel by air or sea to the U.S. under the VWP.

More information is available at http://www.cbp.gov/linkhandler/cgov/travel/id_visa/esta/esta_intro/esta_english.ctt/esta_english.pdf

Other helpful links

http://travel.state.gov/visa/temp/without/without_1990.html