Santa Rosa (Peru)
The island lies closer to the Brazilian/Colombian side of the river but is entirely in Peru. Santa Rosa is a significantly less developed village than either Tabatinga or Leticia.
Movement between all three countries' border towns is unrestricted; but of course, make sure you're stamped into the correct country if leaving the border area. Try to minimize the amount of times you go through immigration: i.e. if entering the region from one country and leaving from another, just get one exit and one entry stamp - even if you visit all three towns in this area. Anything more seems to annoy the immigration officers.
The Peruvian immigration office in Santa Rosa is fairly central and easy to find, just ask someone. The Brazilian immigration office is located some way down Tabatinga's main avenue, Avenida da Amizade.
The Colombian immigration office is at Leticia's airport, so if you're flying in and moving on to a different country you may as well get an exit stamp when you land. Don't wait several days between getting your exit stamp and the next entry stamp; however leaving it one day doesn't cause any problems.
Fast (about 10hrs, US$75 or 200 PEN) and slow (about 2-3 days, US$20-25) boats run upstream to Iquitos. Small ferries run the short distance across the river to Leticia in Colombia and Tabatinga in Brazil.
If on the slow boat, take a hammock (available in Santa Rosa/Tabatinga/Leticia), as beds aren't provided, and something to eat out of and with (food is provided, plates and cutlery aren't - tip: a 2 litre plastic bottle cut in half makes a good bowl).
Trees, mud, a very big river, Friendly Locals, Pink River Dolphins
All three countries' currency are freely traded in this area.
Santa Rosa is the cheapest of the three towns for accommodation.