USA Guide
Entry requirements
Although regulations have been continually tightening up since 9/11, citizens of 27 countries, including the UK, Ireland, Australia, New Zealand, and most Western European countries, visiting the United States for a period of less than ninety days can still enter the country on the Visa Waiver Scheme. The requisite visa waiver form (I-94W) is provided by the airline during check-in or on the plane, and presented to an immigration official on arrival.
However, even with an I-94W form, each traveller must undergo the US-VISIT process at immigration, where both index fingers are digitally scanned and a digital headshot is also taken for file. What's more, all passports accompanying an I-94W must now be machine readable and any issued after October 2006 must include a digital chip containing biometric data (these are now automatically issued by most countries but check). Anybody with an old passport will require some sort of visa for even a short stay in America and anybody planning to stay over three months will: check 
www.dhs.gov for updates and the list of Visa Waiver Scheme countries.
Canadian citizens, used to being able to make an oral declaration, have also had to provide documentation since January 2008, although an enhanced secure driver's license is still an acceptable alternative to a passport. This may change though so again, check for updates.
Prospective visitors from other parts of the world not mentioned above require a valid passport and a non-immigrant visitor's visa for a maximum ninety-day stay. How you obtain a visa depends on what country you're in and your status on application, so contact your nearest US embassy or consulate. Whatever your nationality, visas are not issued to convicted felons and anybody who owns up to being a communist, fascist, or drug dealer.
On arrival, the date stamped on your passport is the latest you're legally allowed to stay. The Department of Homeland Security (DHS) has toughened its stance on anyone violating their visa status, so even overstaying by a few days can result in a protracted interrogation from officials. Overstaying may also cause you to be turned away next time you try to enter the US.
To get an extension before your time is up, apply at the nearest Department of Homeland Security office, whose address will be under the Federal Government Offices listings at the front of the phone book. INS officials will assume that you're working in the US illegally, and it's up to you to convince them otherwise by providing evidence of ample finances. If you can, bring along an upstanding American citizen to vouch for you. You'll also have to explain why you didn't plan for the extra time initially.