This info was taken from the US State Dept website in late May, 2008 and is subject to change.
Panamanian law requires that travelers must either purchase a tourist card from the airline serving Panama or obtain a visa from a Panamanian embassy or consulate before traveling to Panama. Further information may be obtained from the Embassy of Panama, 2862 McGill Terrace NW, Washington, DC 20009, tel. (202) 483-1407, or the Panamanian consulates in Atlanta, Boston, Chicago, Honolulu, Houston, Los Angeles, Miami, New Orleans, New York, Philadelphia, San Juan, San Diego, San Francisco or Tampa.
Brazil: A passport and visa are required for U.S. citizens traveling to Brazil for any purpose. Brazilian visas must be obtained in advance from the Brazilian Embassy or consulate nearest to the traveler’s place of residence. There are no “airport visas” and immigration authorities will refuse entry to Brazil to anyone not possessing a valid visa. All Brazilian visas, regardless of the length of validity, must initially be used within 90 days of the issuance date or will no longer be valid. Americans reentering Brazil must be able to show an entry stamp in their passport proving that the visa was issued within 90 days; otherwise they will not be allowed reentry. Immigration authorities will not allow entry into Brazil without a valid visa. The U.S. Government cannot assist travelers who arrive in Brazil without proper documentation.
Bolivia: Effective December 1, 2007, U.S. citizens seeking to enter Bolivia as tourists must have an entry visa. Bolivian tourist visas can be applied for by mail or in person at Bolivian consulates in the U.S., as well as at Bolivian ports of entry, such as at Bolivia’s international airports and at land border crossings. Bolivian tourist visas are valid for five years from the date of issuance and allow the bearer to enter the country three times in a year for a cumulative stay of not more than ninety days. The cost of the tourist visa is $100 U.S. dollars. The $100 fee can be paid in cash, by deposit to the Bolivian Consulate’s bank account or by money order. Tourists applying for their visa upon arrival to Bolivia must pay this fee in cash to immigration authorities. In addition to the $100 fee, the applicant must present a visa application form with a 4×4 color photograph, a passport with a validity of not less than 6 months, evidence of a hotel reservation or a letter of invitation in Spanish, proof of economic solvency (credit card, cash or a current bank statement), and an International Vaccination Certificate for yellow fever.
Paraguay: A passport and visa are required. U.S. citizens traveling to Paraguay must submit completed visa applications in person or by secure messenger to the Paraguayan Embassy or one of the consulates and pay a fee. Paraguay issues visas for one-entry or multiple entries up to the validity of the U.S. passport. Applicants under 18 years of age traveling alone must appear with both of their parents or a legal guardian. In case of a guardian, an original and one copy of proof of legal guardianship are required. A document of authorization from parents/guardian will be accepted only if it is notarized and certified by the county clerk. Travelers entering or departing Paraguay with regular U.S. passports will be fingerprinted. Effective January 1, 2008, some airlines will begin to include the Paraguayan airport departure tax in the price of the airline ticket. It is recommended that you check with your airline in order to determine whether or not the departure tax has been included. If the tax is not included in the airline ticket then payment would be required upon departure in either U.S. or local currency (no credit cards or checks accepted). Visit the Embassy of Paraguay web site at http://www.embaparusa.gov.py for the most current visa information.
Chile: United States citizens entering Chile for business or pleasure must have a valid passport and visa. Visas may be obtained at the port of entry upon payment of a fee. The visa is valid for multiple entries to Chile and remains valid until the expiration of the passport. U.S. citizens are admitted to Chile for up to 90 days. An extension of stay for an additional 90 days is possible, but requires payment of another fee. Visitors will be issued a Tourist Card upon entry that must be surrendered upon departure. . Visit the Embassy of Chile web site at www.chile-usa.org for the most current visa information.