For nationals of the EU, UK, US, Australia, Canada, Japan, New Zealand many other countries, a visa is not required for entry into Panama, but they need to buy a tourist card when they arrive. The card costs 5US dollars and allows tourists to stay in the country for 30 days. From May 2008, those who fly in are required to pay US $13 for the visa stamp to enter Bocas del Toro. For the stamp, tourists need to show a return ticket, US$500 in cash or traveller's cheques, and yellow fever vaccination if they are travelling from a country which has cases of yellow fever (this includes most of Latin America but not the USA).
Note that clean-cut travellers from developed countries such as the USA can get by the border officials without being subjected to a thorough check.