Are there any countries where citizens are free to use any currency?
Wikipedia has a list of countries which ban foreign exchange use by its citizens.  It's actually quite short but does include India and China. Sometimes economic collapse limits enforcement.  For example, after the collapse of the Zimbabwean dollar (and its government running out of sufficient foreign exchange to buy the paper necessary to print more), the state turned a blind eye as the US dollar and South African rand became de facto exchange. Practicality will limit the availability of foreign exchange even in free-market economies.  The average business can't afford to have a wide range of alternative currencies sitting around.  Businesses which cater to large numbers of addled tourists sometimes offer one or two alternative currencies in the hopes of charging usurous rates of exchange.  Even bureaux de change sometimes require you to order your "rarer" foreign exchange in advance. So, while it may be legal, it isn't always feasible.