Puerto Maldonado -- Quick, easy, and cheap, motocarros are everywhere; most rides in town cost S/2. Ferries cross the Ríos Madre de Dios and Tambopata daily. If you just want to cruise across the river you'll have to negotiate the price, but expect to spend about S/20 per person.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.