The word cited most often is "Polish."  When it is capitalized, it means "relating to or coming from Poland"; when it is lowercased, it refers to the stuff one uses to shine cars, furniture, and so on.

Some less obvious ones exist, however.  As one astute contributor to the Forum noted, another example is "Japan," which, in its lowercased form, refers to "a black enamel or lacqeur used to produce a durable, glossy finish" and to "an object decorated with this substance."