We've told you before how travelling to the US doesn't generally need a visa, but does require you to acquire an Electronic System for Travel Authorization (ESTA) approval. As of September 8, that will no longer be a free service: you'll have to pay $US14 for the privilege.
The US government points out that Australia charges a similar fee for visitors here, so it's not exactly rational to complain. A more practical barrier for some people is that payment will only be possible by credit card.
Regardless, make sure you have a ESTA if you're planning to travel under the visa waiver scheme: if you don't, you won't be allowed on the plane when you hit the airport. Applying for a visa is more complicated and expensive, and so only makes sense if it's a requirement, as it is for journalists and people from countries which don't take part in the waiver program.
You only need to pay the ESTA fee once in a two-year period; after that, it's good for multiple trips. Be sure to check our list of other traps to avoid when travelling to the USA.