Most visitors to Indonesia will require a visa. The only exception are people carrying passports from one of the other member countries of the Association of Southeast Asian Nations (ASEAN), who get a 30 day visa free entry. Passport holders from 36 countries - which includes the USA and EU countries - can apply for a visa on arrival.
Indonesia Visa on Arrival
Tourists holding passports from North American, European Union or Gulf Cooperation Council countries can apply for a visa on arrival. The costs are US$10 for a 7-day stay and $25 for a 30-day visa. Note that the visa on arrival is purchased at a separate counter. Be careful not to queue up for passport control until you have purchased your visa. The process is easy on arrival at Bali, but a little tricky at Jakarta, where the visa counters are easy to miss.