According to Wikiworld there are only two remaining "United States", USA and Mexico. Not so many countries to apologize to for stealing "Unistatians" as to the Americans living in the 35 countries of the two America continents.
Current
The United Mexican States is the official name of Mexico
Historical
United States of Belgium, a confederation that existed during the year 1790
Republic of the United States of Brazil (Portuguese: República dos Estados Unidos do Brasil) was the name of Brazil between 1889 and 1968
United States of Central America (informal name), more properly known as the United Provinces of Central America and the Federal Republic of Central America (through several name changes), a defunct 19th century federation of the nations of Central America
United States of Colombia, name held by Colombia between 1863 and 1886
United States of Indonesia, name of the country from 1949 to 1950
United States of the Ionian Islands, former British protectorate from 1815 to 1864
United States of Stellaland, a short-lived political union of Goshen and Stellaland proper in southern Africa
United States of Venezuela from 1864 to April 15, 1953
