Panama has long used the US dollar as its official currency. Recently El Salvador did so. Guatemala has announced it will permit US dollar accounts and basically allow the dollar as a parallel currency, though it is not adopting it as the official currency. Belize of course for many years has had its dollar, which is worthless outside of Belize, pegged at 2 BZ to 1 US $, and the US dollar is used as a parallel currency.
There has been talk that Costa Rica and perhaps the rest of the C.A. countries will soon go with the U.S. dollar as their currencies.
In South America, Argentina and more recently Ecuador led the way in dollarization of their currencies.
Many of the island nations in the Caribbean use the greenback as either their official or unofficial currencies.
The practical effect of dollarization for visitors, at least in the short term, is likely to be higher prices. Countries with weak currencies tend to dollarize first, and they immediately become more expensive to visit for dollar holders.
Belize First Magazine http://www.turq.com/belizefirst/