All arrestable offences bar travellers from entering the USA under the Visa Waiver Program.
You're officially required to apply for a visa. This can be expensive (there's an initial fee for the application and a further payment for the courier service to return your passport), inconvenient (you have to obtain a copy of your police record, then attend an interview in London; your passport might be retained during a period when you want to go abroad) and time-consuming (some people get their visa within days, others wait many months).
Most people simply 'forget' about their convictions and enter the USA (illegally) under the Visa Waiver Program. The US authorities have no direct access to UK criminal records so the chances of them knowing about your conviction are effectively zero.
To do things 'by the book', start here:
http://www.usembassy.org.uk/cons_new/visa/niv/ add_crime.html
Chris