International drivers permit for driving in the US

http://www.usa.gov/Topics/Foreign_Visitors_Driving.shtml is the State Dept site on the subject. It explicitly says that you'll need to get an International Drivers Permit before entering the US. http://finland.usembassy.gov/faq_pre.html site from the US Embassy in Finland has some more details about how to acquire such a thing, and the fact that - like so many things in the US - the exact length of time you can use your Finnish license while in the US will vary depending on the state you're in.

http://www.drivers.com/article/207/ entrusted with issuing IDPs is generally the main automobile association (or associations) of the country where your licence is issued: in Finland, that's http://www.autoliitto.fi/in_english/international_driving_permits/. As phunniemee says, they're basically an official translation to supplement the licence itself: in the US, an IDP is recommended for that supplementary value, but in most states it's not required in order to drive. On the other hand, car rental and insurance companies may want to see one.

The other situation in which an IDP can be useful is if you're re-establishing residence in the US and want to skip some of the bureaucratic steps that may be required by a particular state DMV -- e.g. if you want credit for your foreign licence, the IDP may remove the need to have it translated. As with all driving-related things in the US, that varies from state to state.

You've heard already what an IDP is from other (http://ask.metafilter.com/198733/International-drivers-permit-for-driving-in-the-US#2860350 and http://ask.metafilter.com/198733/International-drivers-permit-for-driving-in-the-US#2860411). As holgate said, too, the issuing body for IDP's is almost always the main automobile association in the country where your license was issued. However, if the country in which your license was issued differs from your country of residence, you can also get it from the main automobile association in your country of residence in certain circumstances (speak to the association first, then your consulate). An anecdote: the girlfriend and I recently travelled to the U.S. for a road trip. I am British, living in the Netherlands, and she is Dutch. My girlfriend's driving license is in Dutch, and as such she did not need an IDP, but it was recommended. My license is in English, so I did not need an IDP at all (i.e. no translation of my license was necessary).