It's true that allowing a spouse as an extra driver with no fee, varies by company and by location (ie country).
Currently, in North America, of the eight national branded car rental companies (Alamo, Avis, Budget, Dollar, Enterprise, Hertz, National and Thrifty), only Avis, Budget and Enterprise offer NO extra driver fee for the spouse of the renter (assuming that they meet all additional requirements -- age, drivers license, etc).
All of my research indicates that every company outside of North America charges an additional driver fee for the renter's spouse. I've checked with all the major brands, but there may be small, independant car rental agencies in the country that you're looking to rent in that may waive that fee; you would have to check the policy for each individual company/location.