Visa question not answered in the
 

visa section. On a separate website it lists every country in the world as of 12 July 2007 that does not require visas of U.S. passport holders. This includes nearly all South American countries, but most of the posts talking about border crossings mention visa fees, application forms, etc. There are two likely explanations: the first is that most posters are from a country whose citizens pay a visa fee; and second is the officials are collecting the visa fees and riders pay them to avoid hassle. The third explanation might be that the information on the other site is really whacko. Can someone shed some light on this? I don't mind paying a visa fee, but if it is not required, why should I?

Visas
 

Check the relevant consulate website and call your local embassy for any 'problem' countries. For example, Venezuela does not require a 'visa' for US citizens but you do need to apply for a entry permit. Also, sometimes req't for entry by air and land.

I know that Bolivia charges 100$ visa fee for north american tourists, but nothing for europeans.

But most border crossings are free in south america, as far as my experience goes. Central america on the other hand is money on the table all the time...
But not much, so why care.

Many border crossings charge those crossing a genuine admin fee or tax regardless of whether those crossing require a visa or not.