The nationals of 99 countries, including all the usual suspects, will receive a visa on arrival valid for 30 to 90 days. Rules for visiting only Jeju are even more lenient, allowing in everybody except citizens of 18 countries (notably including China and India). See the Immigration Bureau (http://www.immigration.go.kr/) for the latest details. Don't overstay, even by a single day — this incurs heavy fines and possible jail time, and you'll probably be banned from re-entering.